What is the date under "Last Update" column in Rankings Discovery?

It states when the term combination position on this row (Rank column) was last updated (recorded - when was it in the listed position).

With Rankings Discovery, you can find and add term combinations your websites are ranked for, but...

For each term combination found, we tell you its position range. On paid accounts, you'll see...

Enter from the left menu: Tools > Keyword Suggestions. You'll get to the suggestions section,...

We don't know for which combination you track it. From keyword suggestions, you can add the...

Using Keyword Suggestions, you can find and add additional keyword ideas for each of your URLs...