As a pool owner you should understand each piece of equipment on your pad and how water flows through them. When you know what each one does you can better troubleshoot any issues and protect your equipment from damage.
Its easiest to think of your pool as one big loop. The pump sucks water from the pool and then returns it through your equipment, back into the pool. Anything before the pump we call the suction side. The pump will suck water from skimmers and main drains into its pump basket before pushing it out the top. There are usually a handful of valves infront of the pump to let you choose which lines you want to pull water from.
Once the water gets pushed out of the pump it typically goes into a filter. If its a sand filter it will first hit a multiport valve that will direct where water goes. If its in its normal operating mode it will go into the filter and then back out of the return side of the filter.
After filtering the water, many systems will heat and treat the water depending on what equipment you have. From there water is sent back into the pool from the return jets.
Before ever turning a pump on make sure you follow the pipes to undertand the flow of water and ensure that it can freely flow from the suction side of the pool all the way to the return side.