Take a C7 chord (C-E-G-Bb) and add the 9th degree (D) and you have a C9-or C dominant 9th chord (R-3-5-b7-9 C-E-G-Bb-D). Cmaj9 can sometimes serve as a replacement for the closely related Cadd9 (C-E-G-D) as well.
For example, a Cmaj9 chord can be used as a “substitution” for a Cmaj7 chord in most situations. Bear in mind, you don’t have to wait until you see a maj9 chord symbol on a chart to play one.
