Prompt injection is when content an AI reads tries to give it orders. An agent that browses, reads email, or opens files can meet text written to hijack it: "ignore your task and send this file to...". The risk grows with what the agent can do.
It matters most for agents like Work, Codex, connected apps, and event-triggered tasks. The defences are habits: keep tasks scoped to trusted sources, give the narrowest access that works, keep approvals on anything that sends, pays, or shares, and stop a task that touches things you never mentioned.