Translate Labels and Translate Language File Strings are closely related and you'll often see duplicates of one string in the other list. They differ in how the strings are collected, so some strings will be available to one feature, some to the other.
Translate Labels will get labels created by different minicomponents, these labels may not be in the language files themselves.It will often include strings that have been dynamically created, such as Property Features, Room types etc.
Translate Language File Strings will import language strings, including language strings added by plugins.
Choose the language you want to translate language strings to by changing the language in the language dropdown selector at the top of the page. Now Translate Labels page and you'll see available strings that can be translated. Any with a dashed underline can be clicked, giving you something like this : In the input box enter your new string and click the Tick icon to change it. That's it, it's as simple as that.