What is 13.3.1 (17D50) mean.?

13.3.1 = iOS version number

(17D50) = build number

Google the wikis for definitions of each if you need that information.

Sometimes the version number won't change, but the build number will. If the version number changes, the build number will change too.

13.3.1 = iOS version number => This is the "piublic facing" number. It changes when something (even minor) is "visible" to users (may be an additional security feature, not necessarily on UI).

(17D50) = build number => This is more an internal reference, to keep track of every change made to the soft, even when nothing is visible to the user.

Note that those numbers grow each time they change.

iOS 15.4.1