Do you remember that time when you brought a hamster home and introduced it to your family for the first time? It was not part of your family before, but now it is. This is kind of like what happens when a new kind of plant or animal is brought to a new environment where it didn't exist before. This is known as an introduced species.
However, not all introduced species are a good thing. Sometimes, like when the hamster started biting everybody, an introduced species can hurt the other plants and animals in the new environment.
For example, imagine if a new kind of fish was put into a pond. This fish might start eating all of the other fish's food, and the other fish wouldn't have anything left to eat. This could be very bad for the other fish and the entire ecosystem, which means all of the plants and animals that live in that pond.
So, introducing a new species to an environment can be risky and we need to be very careful before we do it to make sure it won't cause any harm.