The standard answer you hear is that by the 2nd law of thermodynamics the entropy always increases and increasing entropy is what defines the direction of time. If you say that you don't understand, then you are in good company: The clever mathematician John von Neumann pointed out that referring to entropy is a secure way of winning any argument, because nobody knows what entropy is. So, refereeing to increase of entropy as the definition of increase of time, says nothing.
If you are interested in understanding irreversibility and why you get older as time passes, you may get wiser by browsing The Clock and the Arrow: A Brief Theory of Time, where a new approach is explored based on an idea of finite precision computation.
The basic idea is illustrated in the picture above showing two sawlike structures on top of each other, which can slide with respect to each other one way, but not the other. Why?
Because, what happens pulling the top structure to the right with a certain force F, assuming the bottom structure is fixed, is that the top structure moves right while being lifted up, and the speed of motion is then determined by the power supplied and the weight of the top structure.
But motion to the left is impossible, because an infinite power would required to lift the top structure
the saw-teeth depth in zero time.
Why is then our lives like a saw gliding on top of another always to the right, towards an inescapable end without possibility to return? This is what the Clock and Arrow book seeks to explain. Take a look if you feel you want to know.