What is the SPDX license identifier at the top of a Solidity file, and is it required?

A one-line machine-readable declaration of the source code's license, e.g. // SPDX-License-Identifier: MIT — optional, but the compiler warns if it is missing.

// SPDX-License-Identifier: MIT

It sits as the very first line, written as a comment. Its purpose is to make licensing and code-sharing unambiguous and automatable — a tool can read exactly which license governs the file without a human parsing a legal blurb. MIT is a common choice because it is one of the least restrictive licenses. Leaving it out does not stop compilation (it is only a warning, not an error), but adding it is good practice and clears the warning.

Go deeper:

• Solidity docs: SPDX license identifier — the compiler's rules for the license comment.
• SPDX license list — the registry of identifiers (MIT, GPL, …) you can declare.