Simple enough, Mirror Writing is writing words, sentences, letters, etc. backwards so that when it is looked at in the mirror, it looks the right way. This can be for any language. This isn't used as cipher too often as it's easy to crack and read, but it's categorised as a cipher because it is technically a substitution cipher.

On an ambulance, the word "AMBULANCE" is written backwards so that, when looking in the read-view mirror, the driver can see the word "AMBULANCE" written out properly.

More left handed-people have the ability to write backwards from birth, and writing letters backwards as a child is quite common. Some people use mirror writing because it is easier--for example, left-handed people writing in a language that starts from left and goes to the right (e.g. English) have a tendency to smudge what they've just written, but mirror writing allows them to start on the right side and go to the left, in which case they're unlikely to smudge the writing.

Leonardo da Vinci is probably the most notable person to use mirror writing in everyday life (in his personal notes).