How complete each language is, at a glance. It opens from the link on the Translations page.
What the grid shows
One column per part of the product (namespace) and one row per language, English excluded because it is the source. Each cell shows a percentage and, where you have changed texts, how many are your overrides.
What counts as translated
A text counts as translated when the language has either shipped text for it or an override from you. The percentage is translated texts out of all texts in that part of the product. A part with no texts reads 100%.
So a high percentage does not mean the wording is good, only that nothing is missing.
Using it
Spot the gaps here, then go back to the Translations list, choose that language and namespace, and tick Only missing to work through them.
It needs the View Translations permission.