It's a metaphor. Hopefully this helps clarify things.

The version from David Foster Wallace’s “This is Water” commencement speech at Kenyon College:

There are these two young fish swimming along and they happen to meet an older fish swimming the other way, who nods at them and says “Morning, boys. How’s the water?”

And the two young fish swim on for a bit, and then eventually one of them looks over at the other and goes “What the hell is water?“

The point of the fish story is merely that the most obvious, important realities are often the ones that are hardest to see and talk about.

full speech: https://www.youtube.com/watch?v=m6p9L3LK_9g

more info

https://en.wikipedia.org/wiki/Ideology

ideologies may function as prepackaged units of interpretation that spread because of basic human motives to understand the world, avoid existential threat, and maintain valued interpersonal relationships. ... such motives may lead disproportionately to the adoption of system-justifying worldviews. Psychologists generally agree that personality traits, individual difference variables, needs, and ideological beliefs seem to have something in common.

so in this sense memes are like little atoms of ideology aka "prepackaged units of interpretation that spread because of basic human motives to understand the world"