"Keeping Prime" questions

I am sort of new to pool upkeep. Bought my current home w/ pool already in place. I helped maintain a pool at my frat house in college, which had a sand filter with a drain and we had to backwash every so often and such. I forget how we got air out if it got in there.

My current pool has a cartridge filter (pool is 12x24 inground). Any time I get air in the lines (usually due to water level getting below skimmer), there is a small valve on top of my cartridge filter that I open and it shoots the air out..I close it after it starts shooting water. Is that what "prime" means, keeping the suction at correct levels vs. having air in the lines (and therefore losing suction)? In my case, getting the air out seems pretty simple..is it harder for sand filters? I see numerous discussions on the topic and was just curious.

Priming refers to the pump. When the pump lid is removed for cleaning or servicing, air enters the pump basket and the pump loses prime. When a pump primes, the pump basket fills with water pushing the air toward the filter and returns. What you are describing is actually bleeding the air from the filter which it should do on it's own but always a good idea to bleed the air after the pump has primed. Also, when the pump is turned off, the water should remain in the pump basket.

Thanks for the explanation. I also forgot to mention that I have a "flooded system" meaning my pump and filter are below the level of the pool (since the equipment is in my basement). Does this help my system keep prime?

Most definitly! You will probably never need to worry about priming. However a water leak is always a possibility.