What does oniichan mean?

Onii-chan is a Japanese name meaning 'older brother'

Here's a formality list:

Onii-chan/Niichan - 3 (informal)

Oniisan/Niisan - 2 (polite)

Oniisama/Niisama - 1 (formal)

Leaving the 'o' off of the start just makes it a teensy bit more friendly, which is always nice :)