I would start with the documentation. Here is an extract from the entry for the Summary Report:
Summary Reports
The Summary Report options show or hide a table that contains the following statistics for each variable: LSL, Target, USL, Sample Mean, various Sigma estimates, Stability Index, Cpk, Cpl, Cpu, Cp, Cpm, and Nonconformance statistics. If there is at least one nonmissing process importance value, an Importance column is also included in the Summary Report. These statistics are calculated using the sigma estimate specified in the report title. The columns for Stability Index, Cpk, Cpl, Cpu, and Cp are colored as green, yellow, and red to indicate adequate, marginal, and poor stability or capability. This color coding scheme matches what you would see in the Process Performance graph.
The entry for the Process Performance Plot explains how the levels are set.