Essentially, it argues that the universe could not be eternal, since this would require an infinite past. And we can’t have an infinite past, because this would require an actual infinite set to be constructed through successive addition of finite elements, since the past is just previously present moments added onto each other. And in the same way you can’t count and reach infinity, only keep counting infinitely, you can’t have an infinite past.
Another way to reframe the argument is that for you to reach the present moment, you’d have to first wait for an infinite number of past moments to occur, which is analogous to waiting for an infinite time to pass, or walking an infinite distance and eventually getting to the end.
I struggle to properly conceptualise infinity, but what is the argument against this?