It means that's how much a movie has made domestically compared to it's domestic opening weekend. The bigger the multiplier, the better "legs" a movie has, and generally means audiences really liked it but didn't necessarily rush out to see it (really depends on the movie). Basically it means that it continued to make a good amount of money for a long time.

So if you take Ant-Man Quantumania's domestic total which is $214,504,909, and divide it by it's opening weekend $106,109,650, you get 2.021 which rounded down is 2x. I hope this helps :)