In a word, meme coins are cryptocurrencies based on online memes and jokes. The Doge meme, for example, was inspired by a viral photo of a Shiba Inu, and Dogecoin was born as a result. These cryptocurrencies aren't meant to be taken seriously, but they can still gather traction if members of the community purchase into the latest one to join in on the joke. When a coin gets popular and its value rises, ordinary investors may jump on board, further increasing the coin's value. Famous online personalities and celebrities also help to enhance the popularity of certain meme currencies.
On Solana, meme tokens are gaining popularity again. A list of them can be found here.
https://medium.com/cryptobrosresearch/new-rise-of-meme-tokens-on-solana-cf67082aff07