Definition of "on to"

on to

preposition
1. Upon; on top of.
2. (informal) Aware of.
  Quotations
  Fortunately “the F.B.I. was on to them” and foiled the plot, but the alleged kidnapping conspiracy, Walter argues, is a harbinger of worse to come.
  2022 January 12, Ross Douthat, “Let’s Not Invent a Civil War”, in The New York Times‎
3. Used to indicate, or signpost, logical progression to a new topic in a talk or discourse.