Learner's definition of MOSTLY
— used to say that a statement you are making is true or correct at most times or that it describes a usual situation or condition
He gets around mostly [=mainly, chiefly] by car. [=he usually uses a car to get around]
“What did you do this weekend?” “I worked in the yard, mostly.” [=I spent most of my time working in the yard]
I concentrated mostly [=mainly] on getting good grades when I was in college.
almost all or almost completely