Suspension of disbelief means that there are some unusual premises which normally you might not be willing to accept, but in order to enjoy the story you go along with it.
A good example of this is Harry Potter. This is a story involving magic. Magic does not exist. However, it is a really fun story full of action and adventure, with themes like friendship and courage. It is worth it to go along with the idea that magic exists so that you can enjoy the story. It is also a lot of fun to think about magic.
Sometimes story lines stretch believability. For example, some story lines have huge coincidences that you have to accept in order for the entire rest of the story to work. If you don’t accept that coincidence as plausible, you will not enjoy the rest of the story. For example, in Star Wars, what is the likelihood that Luke’s Uncle Ben would buy R2-D2 and C-3PO, the very droids that his father had owned? This sets off a chain of events that leads Luke to Obi-Wan Kenobi and his destiny.
In movies, suspension of disbelief helps a viewer appreciate and enjoy a plot line. Sometimes if you think too hard about something, it saps all of the fun out of it. Just sit back and enjoy the ride. Movies can be just entertainment.