A checkpoint on every sign-in to Microsoft 365 that can block it, allow it, or ask for more proof.
Like a doorman who checks your ID, your name on the list and whether you came in the right door, not just your ID.
A staff member types a code into a fake Microsoft page. Without Conditional Access, the attacker's computer is accepted. With it, the sign-in is refused because the computer isn't approved.
Cloud covers setting these rules once your licences include them.
We already have MFA. Why do we need this?
Because MFA checks the person, and Conditional Access checks the whole sign-in. Attackers now trick people into completing MFA for them.

Every WILDCARD card exists to solve one named problem. Find yours.