The number below each date is your Days Remaining. It shows how many days you can stay in Schengen starting from that date - with all your previous trips accounted for - without exceeding 90 days in any 180-day period.

The 90/180 rule is checked every single day of your stay, not just at entry. Each day, the system looks back 180 days (which means the start of that 180-day window moves forward by one day, every day) and counts how many days you’ve already spent in Schengen - and the calendar shows how many days you may stay before any 180-day period would contain 91 days in Schengen.