What are GFCI Receptacles
Simply put, they protect you from shock hazards.
A Hazard can be considered as a situation.
Think about it like this.. The GFCI is a computer that senses something is not right with the appliance that you may be using and decides that for your safety, I'm not going to let you use this
because you may get hurt.
This is not to say that the appliance is bad, or won't work in an unprotected socket, but it's not safe, so I won't let you use it.