Defender - Quarantined Items

Quarantined items are programs and/or data that is suspicious. When something is put into quarantine Defender will give you an alert level and date. It is up to you to decide if you want to keep or remove the item from your computer.

The reason why Defender uses the quarantine step is because of what is called False Positives. This is where a potienal infection is removed, but in actuality it is not a dangerous program. This way you can tell Defender and your computer it is a safe program, thus you are accepting the risks.