Here are a few examples:
If it is raining, the ground is wet. It is raining, so the ground is wet.
If you are human, you are also a mammal. You are a human, therefore you're a mammal.
If you get 3 strikes, you're out. You have three strikes, so you're out.
Each example follows the same logical structure. If (something happens), then (something else will happen). Since (the first thing happened), I know that (the second thing happens). I know that the ground is wet because it is raining. I know that I am a mammal because I am human. I know that I'm out because I got 3 strikes.
Keep in mind that this does not work in reverse. I do not know that it is is raining just because the ground is wet (someone could be washing their car). I do not know that I am human because I'm a mammal (I could be a horse). I do not know that I got 3 strikes because I'm out (I could have hit a pop fly). If I were to make an argument this way, I would commit the formal fallacy known as affirming the consequent.
The consequent is the Q part of "If P, then Q." This fallacy effectively says, "If P, then Q. I know that Q is true, so P must be true." As shown in the examples above, this isn't true. There could be other reasons Q is true without P being true.