What is "forced" or not depends a lot on who you ask.

But in general, the term means that a cast is diverse for the sake of being diverse. This is especially true, if the casting ignores other context.

For example: Casting 3-5% black people in a British soap is just "diversity". That would be roughly a representation of UKs population, so it fits the setting and any "rules" within the fiction.

Casting 5% black people in a series about the Vikings would be perceived as forced diversity. Because black people generally weren't around in Scandinavia during the middle ages.

The term can also apply when an actor is cast to add diversity, rather than casting the best actor for the role, even if it otherwise fits the setting. I can't think of any examples where I personally feel this way though. But I think some people felt this way about Rey in "The Force Awakens".

Of course the term also gets thrown around a lot by people who feel unhappy about how "social justice" and "political correctness" influence our society and thus media. Those people still see it as "forced", but are rather trigger-happy to label any non-white and non-male casting choice "forced diversity". This can make discussions about the topic a little difficult.