Shoulder Surfing is the act of a thief who tries to view and steal your personal information while standing behind you when you use devices like computers, ATMs, Card payment machines etc.
You can be at higher risk of shoulder surfing if you use public Wifis or use similar passwords for more than one personal account.

Example:
Let’s say you’re at an ATM machine and you need to withdraw money. You enter and start typing your PIN number on the keypad. Now, lets say there is a person standing behind watching you do it. They might even take a photo or video of you entering your PIN on their phone without you realizing it. Once they have your PIN, they can use it to withdraw money from your account.