Incidentally is a word you use when you've got more to say on a topic or want to transition into a new subject. Think of the word incidentally as a more formal way to say "by the way."
Incidentally can also mean "by chance" if you want to remark on a coincidence. Sometimes, things just happen by chance, like you and your friend showing up at the same place incidentally. Incidentally, the word ends in lly so don't forget to throw that extra l in there!