OP_RETURN is a Bitcoin script opcode used for marking transactions with a bit of extra data – kinda like leaving a note on them. It's how you make inscriptions, actually.

Inscriptions are just the messages or data bits that you attach to Bitcoin transactions using OP_RETURN. They can be anything from some text to a hash or even tiny bits of code.

Think of OP_RETURN as the pen, and inscriptions are what you write with it!