Technical meaning: 75 tokens (attributes or concepts) make up a chunk that goes to CLIP. Whenever you write a prompt that exceeds 75 tokens, you'll notice that the limit goes up to 150, then 225...using BREAK tells it to end a chunk short and start a new one. A lot of people can tell you the technical meaning, but only a guess to what that does in practice. I am one such person.
Practical use: People use it to try and get more consistency in their output, by dividing the prompts into different subjects. Such as "Person, description of person, description of clothing BREAK environment, description of environment"
I tested this with the prompt: "Girl with blue eyes wearing orange dress, sitting on couch, in room with window and curtains." Many times, the couch and curtains would also be blue/orange,

When I used the prompt: "girl with blue eyes wearing orange dress BREAK sitting on couch, in room with window and curtains," the couch and curtains were free to be different colors. They could still be blue and orange, but not ALWAYS.
Doesn't work as well when you try using BREAK between different color clothing, or different characters in a single scene. The more chunks you use, the more memory-intensive the operation becomes (somebody please correct me if I'm wrong about that), so don't get too carried away with BREAK.