Proper grammar is the set of standard rules for building sentences and choosing words, the form expected in writing and formal situations. She don't like coffee breaks it; She doesn't like coffee fits a job application. Casual talk may sound different and still work.
Proper grammar means putting words together the way standard English expects, so the sentence is complete and clear. Went to the store after work is a fragment, since no one is mentioned as going; We went to the store after work is proper, a full thought with someone doing the going.
A run-on also breaks it. The bus was late we missed the meeting jam two sentences together; proper grammar separates them: The bus was late, so we missed the meeting.
Proper word choice counts too. I put the keys over there on the table uses the right word, since there points to a place. I put the keys their on the table mixes up their and there, a common improper choice that careful writing avoids.