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, and so you can use the word in that case too. Incidentally, the word ends in lly so don't for get to throw that extra l in there!
