What is a browser extension???

So I'm newer to this space and understand how an electronic wallet works but not how an actual browser extension like metamask works. What does metamask do that another ether wallet can't? I understand you cant get on an eth website by googling it but can exodus, atomic, MEW and all the other wallets do this as well or is there a keyword I'm looking for to make sure my funds dont get lost interacting with smart contracts?

Any help/feedback appreciated!!!