To summarize for posterity:
Once you activate a Journey, your members' activity in Slack will begin to trigger milestone checks.
During each milestone check, Tightknit will evaluate all ingested activity data that occurred during your Journey's start date / end date window.
As an example, let's say Bob sent 4 messages yesterday. Then you activate a Journey with a milestone for 5 messages sent, within the last month. The next time Bob sends a message, it triggers a milestone check. Tightknit checks that Bob has now sent 5 messages within the Journey window and awards him the milestone.