Shoulder surfing is a term used in cybersecurity and it is often assumed that everyone understands the term
Shoulder surfing is a way that people will quite often steal passwords. They could be passwords for your computer, for programs, for your debit card PIN or ATM card PIN (PIN is Personal Identifying Number, the number you type in as a password at checkout or at an ATM).
The way people steal password when shoulder surfing is looking over your shoulder while you type in a password (where name came from) or watching to the sid of you for what you are typing in. This is the reason you see most debit card machines with shields on them so the next person in line does not see what you type.
To protect yourself from shoulder surfing, always keep the area you are typing in shielded or covered. Trying to keep people standing back further can be helpful but in a lot of cases in stores the line is pushing up on you. When you are the next one in line, show courtesy and stand back a little when some uses a debit/credit card.
Shoulder surfing is considered a part of social engineering which are items where people voluntarily give security information to the bad people, although in some cases like shoulder surfing, it may be done with out realizing it.