Cache in general is a type of temporary and quick access data used by apps. Any temporary information created or used by an app is part of cache.
In general, cache shouldnt cause problems, but occasionally it can become corrupted and cause an app to misbehave.
In general, no, it's not dangerous to clear it, but by doing so, you force the app to recreate any information it had previously stored in cache, which can slow down the app until the information is recreated.
Unless you're having problems, it's really not something you should ever have to mess with