These are words that sound the same but are spelled differently. For example, cite, sight and site; peek and peak; no and know. Computer spell-checkers don’t catch incorrect homophones, and writers who overlook them can produce comical results. For instance:
“People searched endlessly down isles full of office supplies.”
“Make sure to use extension chords properly.”
My advice: Whenever you run across a homophone in writing or editing, stop, think and look it up. You are always better off taking a little extra time and looking something up than in looking like a fool by publishing something you thought was right.