BMR as it relates to caloric expenditure is a percentage of the total caloric output of the person for 1 day.
People expend calories through a couple different ways:
1) life supporting functions (respiration, heart beat, etc)
2) Daily movement (work, walking, eating, etc)
3) Exercise (workouts, possibly teaching, etc)
To make it simple BMR is the #1 out of those 3.
Hope that helps!