Imagine you have two groups of friends, the "red team" and the "blue team". Each team has its own set of rules and games that they play. But one day, the teams decide to come together and form a new group called the "purple team".
Now, the purple team has some rules from the red team and some rules from the blue team. For example, they might use the red team's rule for playing tag but the blue team's rule for scoring points.
This is kind of like a semidirect product. You have two groups (the "red team" and the "blue team") that have their own unique properties and rules. But when they come together, they form a new group (the "purple team") that has some properties from each of the original groups.
In math terms, a semidirect product is a way of combining two groups (let's call them G and H) to form a new group (let's call it G⋊H). The resulting group has some properties from G and some properties from H, but it's not exactly the same as either of the original groups.
Just like how the purple team had some rules from the red team and some from the blue team, a semidirect product group has some elements that behave like the elements in G and some that behave like the elements in H. But there are also some new rules that apply to the entire G⋊H group that didn't apply to G or H individually.
Overall, a semidirect product is a way of combining two groups to form a new group that has some properties from each of the original groups.