Web3 libraries
Ethers.js is one of the most popular libraries for interacting with the Ethereum Blockchain and its ecosystem. Smart contracts that are deployed on the Ethereum blockchain have functions that can be called externally by any other account on Ethereum, be it an externally owned account (EOA = user wallet) or another smart contract. Many of these functions require certain parameters to be fed into them, they also rely oftentimes on an external state like prices of tokens on the blockchain, balances of the user’s wallet, and more. Ethers.js is what allows a user interface to call these functions, users can input certain information in the frontend of your application and that information can be put into the function call of the smart contract, after the transaction is broadcasted the EVM will try to execute that function call and if every check inside of the function doesn’t give out any errors it will execute, otherwise, the transaction will revert.
Ethers.js is currently the most popular Ethereum library among developers, but there are alternatives like web3.js, web3.py, Brownie, and many others. The second most popular framework is web3.js and it is the Ethereum JavaScript library that has been around the longest. For a comparison of ethers.js and web3.js read this article written by the Moralis developer team.