STARTTLS upgrades an existing plaintext protocol connection to TLS after the server advertises support. In SMTP, a client connects, issues EHLO, sees STARTTLS, negotiates TLS, then normally issues EHLO again.
Opportunistic SMTP TLS improves confidentiality but does not alone guarantee that a sender will refuse delivery when TLS is unavailable or unauthenticated. MTA-STS or DANE can add stronger policy.