As others have commented, part of the beauty of this play lies in the fact that you can attach so many different meanings to the action or inaction of the play. Critics have variously argued that the play could be about almost every topic under the sun thanks to the way in which the play leaves itself open to interpretation. Universal meaning can be applied to so many aspects, but in particular the whole concept of waiting and what it means to wait and the identity of Godot.
The play is an existential study in action. Beckett shows us what happens when people are breathing, but not living. Vladimir and Estragon are waiting for Godot in the hopes that Godot will tell them what to do and therefore bring meaning to their lives. They have so little curiosity or motivation beyond their day to day existence: one man his hat, the other his boots, that the audience is left to think harder about what we do observe and what it means to be existentially dead or existentially alive.
Ah, the work where nothing happens, but everything happens! :) Yes, we have two characters sitting in one another's company seemingly doing nothing all day for several days...however, when we look past the apparent inactivity, we have the flurry of thoughts, the welter of conversation and connections, the chaos of every day living in a calm and almost apathetic setting. There are layers of relationship, and just existence being analyzed in this short play. This sort of thing happens daily in our own lives, yet we don't take the time to absorb it. Facial expressions, body language, ads, technology, communication of all kinds--visual, aural, sensory--delivers a message of one type or another.
This play is a veritable onion to be unpeeled in layers and used to study human interaction, the workings of the mind, and the life of the day to day.
The above statement has much in way of connection to Beckett's Waiting For Godot. The context of these two characters waiting for a dinner guest who is never to arrive holds so much meaning for the readers who exist outside of this context. Beckett's work details the essence of what lies at the heart of the human condition in terms of the concept of "waiting." Part of the reason that the work is "one of the most important works of our time" is because it speaks to this idea of "waiting" and what it means to "wait." Beckett is wise enough to not make a didactic statement about waiting, but rather asks the audience to observe what is happening on stage and in the text. After the lights dim or the book is put away, the reader or audience must make critical assessments about their own life and what they just read/ saw. It is in this light where the idea of contextually specific observations can have universality. Interestingly enough, one of those contexts that are specific concerns the friendship of Vladimir and Estragon. As the audience experiences their friendship, one actually mirrors the same contextually specific element in discussing the work. Waiting for Godot is a work of literature that has to be discussed and analyzed in a collective element. The full force of the work cannot be felt in an isolated context. In this, the reader/ audience becomes similar to Vladmir and Estragon in their own intense, yet offbeat discussions. In this light, Beckett has transformed the audience into the same context and conditions as his characters. In this, the universality of the people with whom we wait and with whom we live is something reaffirmed through the work into a context outside of it.
I am also doing this question, but in relation to Heaney's poetry. I would note that the definition of 'universal human interest' is problematic, and far from universal, in fact the definitions are so wide-ranging I would question the existence of 'universal human interest', think about what the term would mean from the point of view of a Marxist, an anthropologist, or any number of philosophical perspectives.