Two independent clauses joined together without proper conjunctions or punctuation.
Separate them into two sentences with a period, use a semicolon (;), or use a coordinating conjunction (for, and, nor, but, or, yet, so).
I love cats they are cute.
I love cats BECAUSE they are cute. / I love cats; they are cute.
Check if your sentence is too long and contains multiple independent ideas that should be separated.