I don’t understand the angle limit?

That is indeed rather confusing.

The difficulty is probably mostly, due to the way the Angle between two Faces is measured. It is easiest to look at the Angle between the Face Normals, like so:

When that Angle is larger than the set Angle Limit, the Faces get ‘split’ in the UV Editor. So, in the above example, with an Angle Limit of 90°, the Faces on the left will be one Island and the Faces on the right will be split into two Islands.

(Practically, all those methods, apart from Unwrap, will rarely be used.)