It is really not any harder to define than "freely". Presumably by "what I signed up for freely" you mean "what I signed up for without any coercion, threat of violence, etc". The people using "exploitation" here just mean that those conditions also include the implied threat of not having money to live. This is a real material condition which affects what people are prepared to agree to (even if they might be able to find a better offer by shopping around).

It is not hard to understand, and I suspect you are not trying to understand it.