This means the last or most recent time a score has been updated. Only marketing activities that add or remove points update this date. Activities worth 0 points leave it unchanged.
Last score update is separate to the marketing score.
Example
You filter contacts who have 50+ marketing score AND have a last score update within the past 90 days.
This will include contacts who have 50+ points in total, which can be beyond the past 90 days.