Both TLS and SSL are secure, the difference is how they work.

*) With SSL, the port number you open can be used for secure connection, and secure connection only.
*) With TLS, the port number can be used for both secure and insecure connections. Usually client connects to port first, then issue command LIKE 'STARTTLS' to establish secure connection. If no such command issued, the connection is plain and insecure. This way we don't need 2 port numbers for insecure + secure connections.

Take SMTP and submission protocol for example. Let's say your server supports sending email through both insecure connection (port 25 without TLS) and secure connection (port 25 with TLS).

Here's how to send email through port 25 without TLS:

ehlo test.com
AUTH PLAIN AHBvc3RtYXN0ZXJAYS5pbwB3d3c=
mail FROM:<postmaster@a.io>
rcpt TO:<postmaster@a.io>
data
[...detailed message here...]
.

And here's secure connection (port 587 with TLS):

ehlo test.com
STARTTLS
ehlo test.com
AUTH PLAIN AHBvc3RtYXN0ZXJAYS5pbwB3d3c=
mail FROM:<postmaster@a.io>
rcpt TO:<postmaster@a.io>
data
[...detailed message here...]
.