When someone says a song has a "good beat they are Referring to the

This beat has a catchy tune and that's it.

Explanation:

When a song has a good beat it means that that song is not a masterpiece. It is a simple beat with a 4/4 time signature that is constantly repeating. This beat is not so interesting for exquisite music taste but it is good for someone who likes to dance with the beat, not to feel the emotion in the song.

It makes people happy and cheerful. Te most famous genres of this beat are hip-hop, dance-pop, RnB. One motive is constantly repeating during the few minutes of the song.