Sometimes a published version has to stop being used: it has a security hole, or it was built wrong. Withdrawing is a deliberate act with a rule about what happens to people already running it, and the rule is the part that decides whether the fix works.
About 1 min of reading
Withdraw one.
The mark
Before you run it, say out loud what it will print. Then open the answer.
Read the first example one line at a time. A line that prints shows what appears on screen; the rest set things up quietly.
The withdrawal has to appear in the changelog and in the resolver's message. A withdrawal known only to the person who did it is a fix that reached nobody.
The message names the version to move to. A person reading 'withdrawn' has to have somewhere to go, and the path is the difference between a warning and an action.
A withdrawn version stops being chosen for new installs and stays resolvable for locks that name it. That is what lets an existing build keep working while the bad one is no longer picked.
The withdrawal carries a reason and the replacement. A withdrawal with no reason is a version that disappears from a search, and the next person cannot tell whether they are affected.
Deleting a published artifact breaks the builds that already reference it and makes the history unreadable. Marking it withdrawn keeps both, and marking is the act.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.