This is the most recent data collection with this domain’s ad running on the keyword. Even though it’s close to “the date it was run” that’s not exactly right. We collect data across different parts of the month, so whether it came from June 1, June 6, or June 14, we refer to that set of data as “the June release.” (This does not apply to weekly rank tracking.)
This stat appears when you export a list of keywords into CSV. In the column (last_seen_date) you will see a series of numbers like “20160601.” This code is the date in reverse order 2016 06 01 — meaning the June 2016 release.