In exploit development parlance, a gadget is a block of assembly instructions executed outside their intended order by an attacker-induced control transfer. A gadget might start in the middle of a basic block, for instance, and be invoked when an attacker uses a memory corruption vulnerability to overwrite a function pointer with an address they control.

"Return oriented programming", which is kind of a dumb name, is the idea of harvesting gadgets from the text of a program and then using them as primitives for a new program. Gadgets are stitched together by the "return" instruction (hence the name ROP). When used by attackers this way, "ret" isn't really "returning" so much as it's being used as an arbitrary indirect jump mechanism.

It's a reference to this class of vulnerabilities: https://en.wikipedia.org/wiki/Return-oriented_programming