Considering e.g. Newton's laws of motion, you take a system that is a function of time , e.g. the position of many point particles, and then you reverse the speeds of all particles, then is a solution to that.
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Time reversibility is a concept in physics that refers to the idea that the fundamental laws governing the behavior of physical systems do not change if the direction of time is reversed. In other words, a time-reversible process is one where the sequence of events can be reversed, and the system can retrace its steps back to its initial state. In classical mechanics, many physical processes exhibit time reversibility.