What it's for
Port 995 is POP3 wrapped in implicit TLS — the encrypted counterpart to plain POP3 on port 110. It provides the same "download messages from the server" behavior POP3 is known for, but ensures the connection (including credentials and message content) is encrypted from the outset.
Protocol basics
The underlying POP3 command set and behavior on port 995 are identical to port 110 — retrieve, list, and delete messages using the same simple commands. The distinction is entirely at the transport layer: TLS is established immediately when the connection opens, rather than being an optional upgrade negotiated mid-session. Any mail client configured for POP3 access to a legitimate provider today should be using port 995, not plain port 110, given how little extra configuration the encrypted variant requires.