SPL token
In short An SPL token is a token on Solana created through the shared SPL Token Program, with each holder's balance kept in a separate token account.
SPL stands for Solana Program Library, and an SPL token is any fungible or non-fungible token issued on Solana through its standard token programs. Unlike Ethereum, where each token is its own contract, most Solana tokens are created by one shared program. A new token is just a mint account with settings like decimals and authorities.
Each wallet holds a given token through an associated token account, a small account linked to both the wallet and the mint. Creating it requires a refundable rent deposit in SOL, and fees for transfers and swaps are paid in SOL. The mint has a mint authority that can create new supply and an optional freeze authority that can freeze any holder's token account. A newer version, Token-2022, adds extensions like transfer fees, transfer hooks, and permanent delegates.
For anyone buying small Solana tokens, these authorities are the main thing to check. If the mint authority is still active, the creator can print more tokens at will. If the freeze authority is active, they can lock your balance so you cannot sell. Token-2022 extensions can add hidden taxes or let a delegate move tokens out of wallets.
Before buying, look up the mint address on a Solana explorer and confirm that mint and freeze authorities are revoked, or that you understand why they are not. Check whether the token uses Token-2022 and which extensions are enabled. Look at holder concentration and whether the liquidity pool tokens were burned. Copy the mint address from a trusted source, because names and tickers on Solana are trivially copied.