Browser extensions are little pieces of code that run in your web browser. They can do lots of things, but they mostly just help you navigate the web more easily. One popular example is an ad blocker — it's a piece of software that stops websites from showing annoying ads to you. Another example is a password manager — it stores all your passwords so you don't have to remember them.
Extensions can also come in the form of cryptocurrency wallets. A wallet extension helps you store and manage cryptocurrency (and NFTs) directly in your desktop or laptop internet browser. It also allows you to interact with web3 dapps, sign transactions, and execute smart contracts using your browser's native user interface.