The tiles show the grade trend over successive snapshots and can give you an instant indication about whether your application / applications are improving or worsening for the particular measure.
If the measure schema contains more than 1500 snapshots, then trends section is not displayed. In such cases manually change the snapshotTreshold value to some higher value and restart tomcat. Refer: Health Dashboard json configuration options#Optiondescriptions
Drill down behaviour
Clicking any of the Health Measure trend tiles will drill down to show the trend of the grade of the selected Health Measure in a line graph for each Application in the dashboard. Note that the Average "app" listed in the Displayed Apps panel displays the average grade of all other applications in the dashboard.
Clicking any of the Health Measure trend tiles will drill down to show more detailed information about the specific Health Measure and the trend over successive snapshots. This section is explained in more detail in Health Dashboard - Available information - Overview section:
Critical Violations trend tile