The exact thing you want the reader to do next. One per piece, clearly stated.
A CTA is the specific next step. Click a button. Fill a form. Call a number. Reply to this email. Good CTAs are specific ('Book your call'), one-action-only (not a menu of options), and repeated throughout longer pieces. Ambiguous CTAs ('learn more,' 'get in touch') confuse the reader and kill conversion.
Most marketing fails at the very end. You spent hours on the copy and then used 'Learn More' as the button. Hit the CTA harder than you think you should. The CTA isn't where you're polite, it's where you're direct. Tell them exactly what to do and why now.