The pool filter function obviously is to filter dirt & debris from the pool.
Once the filter becomes saturated or full of dirt it must be cleaned to
restore it to 100% efficiency. The filter is set to backwash & uses pool
water to flush the dirt out of the system. Some filters have a bump handle
on top to dislodge the debris while backwashing. You'll need to look on the
filter to see if it is a sand filter or a DE filter. Sand would just need to
be backflushed when the pressure builds. The DE filter would require the
addition of new DE after cleaning. If needed you'll have to add water to the
pool to maintain the proper level. The level is usually indicated on the
skimmer box(inlet). Look at the frame around the skimmer you should see
arrows on the frame indicating the proper water level. Hope that helps a
little.
Mike