Exploitation means using things that are available in order to benefit from it. It is usually used in a bad sense, although it can be used in a good sense as well.
If a person exploits a situation it means that they realize that they can use the situation to do something which will be good for them. Very often this also means that it will be bad for other people.
People, especially proletariat can be exploited. This means that someone else uses the people's knowledge or skills in order to make them do something for them. Socialists (or communists) often talked about the "exploitation of the masses" when they were saying that Capitalism was bad. They meant that a few rich people were using all the working people to do hard work for them so that they themselves could become rich.
Exploiting people still goes on today.