The full form of VFX is actually just "visual effects", but the term is used to refer to any kind of special effects used in filmmaking, in order to add additional layers or alteration to live filmed footage.
A popular form of VFX is CGI, which stands for 'computer generated imagery', and although most VFX is done through computer technology, technically a computer is not always necessary for a technique to fall under the VFX label.
The main reason for using VFX in film productions is to simulate or recreate environments or sequences that would be either too expensive or too dangerous to film live.
I'm sure you can think of many popular movie scenes which fall into that category!
