In a word, meme coins are cryptocurrencies inspired by memes and online jokes. Dogecoin, for example, was born out of the Doge meme, which was inspired by a Shiba Inu photo that went popular. Although these cryptocurrencies aren't meant to be taken seriously, they can nonetheless gather traction when people of the community buy into the latest one to join in on the joke. When a coin gets popular and its value increases, retail investors may jump on board, further increasing the coin's value. Famous internet figures and celebrities also help to enhance the popularity of certain meme currencies.
On Solana, there is a new surge of meme tokens today. Here's a list of some of them.
https://medium.com/cryptobrosresearch/new-rise-of-meme-tokens-on-solana-cf67082aff07