Hypothetical syllogism is like when we play a game of "if-then". Let's use an example.
If it rains, then the grass will be wet. (If A, then B)
If the grass is wet, then we can conclude that it rained. (If B, then C)
Therefore, if it rains, then we can conclude that the grass is wet. (If A, then C)
In simpler terms, if we have two "if-then" statements and the second statement agrees with the first one's conclusion, we can put those two statements together to come up with a conclusion.
Does that make sense, kiddo?