The meme refers to the notion where an observation (i.e. interaction) collapses the wave function to a single value. As in, prior to observation, a system in a quantum superposition is said to be "in multiple states at the same time", and after the obsevation only one state exists, while all other possibilities are gone (or exist in other worlds, according to one of the interpretations of quantum mechanics [0]).

So, in that meme, the guy looks at one girl (the observed state) and "ignores" all other girls (all other possible states).

[0] https://en.wikipedia.org/wiki/Many-worlds_interpretation