Infinity is the perplexing idea which basically refers to something that grows without bound, no matter what number you might assign to something, infinity would be larger than that number. We make use of infinity in a number of ways in physics. We image that space conceivably could go on infinitely far. We imagine that in principle the universe could go on for an infinite matter of time. But in our calculations, if infinity turns up as the answer to something that we could directly measure with a piece of equipment, then we know that our calculation must be wrong. We have used that as a diagnostic over many decades to tell us if you find infinity popping out of your equations for something you can measure, you'd better go back, think about those equations, modify them in some way in order to get a finite answer is that's the only kind of answer that we could ever directly measure.