To have a "willing suspension of disbelief" means to be able to forget what you know to be true and accept things that are not realistic according to science. This ability helps Mrs. Murry, who is a scientist. As a scientist, she is trained to test ideas, or hypotheses, and decide if they are true or false based on physical evidence. However, by suspending her disbelief, she is able to accept and believe things that she cannot prove. She is able to feel comfortable with the unknown. As a result, Mrs. Murry seems very relaxed when strange things happen to her family, such as the disappearance of her husband, her youngest son's strange ways, the arrival of the strange beings Mrs. Whatsit, Mrs. Which, and Mrs. Who.

You are talking about the "willing suspension of disbelief". The romantic poet Coleridge came up with the phrase to explain how a reader can engage with a text even if the events or themes are not strictly realistic. The reader is encouraged to overlook the plausibility of events and just appreciate the story outside of reality. So, happily leave off what you know to be true.

As a scientist, this allows Mrs. Murry to be open to ideas that current science might not have answers for. She is more willing to entertain the idea of a tesseract, for example.

What is a “willing suspension of disbelief” in A Wrinkle in Time? How does having a willing suspension of disbelief help Mrs. Murry?

I think the phrase you are referring to is “a willing suspension of disbelief.” On its own, this is the idea that we can suspend or set aside our logical and realistic thoughts and ideas in order to embrace something that is outside our experience. It might help to think of it as it relates to attending a theater performance; you know that what is happening on stage is not real, but the actors and crew invite you to suspend your disbelief in order to enter into the world being created on the stage. This suspension allows you to feel and experience what is happening as though it were real. Without suspending your disbelief, you cannot try to experience what you don’t quite understand, because you are bound by your past ways of believing.

There are numerous examples of this in A Wrinkle in Time; so many, in fact, that one could argue it is a theme of the story. Calvin sets aside what he knows of the Murry family, and this allows him to make friends with them and be taken on the journey. Meg tries to understand the tesseract, believing for the moment that there is such a thing as the fifth dimension. Charles Wallace seems naturally disposed to see things that are beyond him, such as when he is able to almost understand the music that Mrs. Whatsit shows them on Uriel.

Mrs. Murry, as a scientist, understands very well that there are things that she does not comprehend. Seeking answers to questions is her business. She also knows that just because she doesn’t have the answer does not mean that something does not exist or is not true. In chapter two, she says to Meg, “I don’t understand it any more than you do, but one thing I’ve learned is that you don’t have to understand things for them to be.” This lesson helps her to accept things that happen. Letting go of the tension between what she can see and what she can believe allows her to have hope, and that sustains her day-to-day living.

L'Engle, Madeleine. A Wrinkle in Time. Square Fish, Farrar Straus Giroux, 2018.

What is a “willing suspension of disbelief” in A Wrinkle in Time? How does having a willing suspension of disbelief help Mrs. Murry?

The correct phrase is "a willing suspension of disbelief," and it comes from the poet Coleridge, who by this phrase meant that people have an ability to accept what seems to be impossible. We are able to think and believe beyond what the rational world says is true. This allows us to engage imaginatively in science fiction, fantasy, horror, Gothic and other non-realistic forms of literature. 

In Mrs. Murry's case, she is a scientist, which means she believes firmly in a rationalistic universe where nature works according to the rules of physics, and everything that happens can be logically verified. In her situation, the willing suspension of disbelief allows her to believe in what Mrs. Whatsit, Mrs. Which and Mrs. Who are telling her about space travel, the reality of tesseracts or wrinkles in time, and what has happened to her husband. Without this ability to "think outside of the box," the scientist in Mrs. Murry would have to reject what she is hearing as impossible.