Solidity (Overview)
Solidity is the most widely used programming language for writing Ethereum smart contracts.
In depth
It looks a lot like JavaScript and compiles to bytecode the EVM can run. Because deployed contracts cannot be patched like normal software, Solidity developers rely heavily on audits and testing. Most tokens and DeFi protocols you interact with were written in it.
Why it matters
Knowing the language exists helps you understand audit reports and why contract bugs are so serious.