It means the gallons per minute the pump is rated for. If your toilet fills at 5 gallons per minute and your pump is a 7 gl/ min pump, your water pressure will suck the second the pressure tank dips. If your pump has a decent flow rate you are looking at a piping issue, a restriction somewhere. Remember, the pipes have to be able to deliver the water fast enough to make you happy. Make sure your feeder pipes are large enough, say, 3/4 inch or larger, and remain that big until they split off to the various faucets.
It should be noted, a pressure tank will only fix very short burst pressure issues, and will not fix a shower and flushing toilet situation, only a better pump, less restrictive piping can fix that. The tank is essentially bypassed in long run examples, like showering and sprinklers or spigots turned on for more than a few seconds.
The pressure tank is there to extend the life of your pump and even out pressure, not increase pressure.
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