Retiring metric represents a Pipeline Slots fraction utilized by useful work, meaning the issued uOps that eventually get retired. Ideally, all Pipeline Slots would be attributed to the Retiring category. Retiring of 100% would indicate the maximum possible number of uOps retired per cycle has been achieved. Maximizing Retiring typically increases the Instruction-Per-Cycle metric. Note that a high Retiring value does not necessary mean no more room for performance improvement. For example, Microcode assists are categorized under Retiring. They hurt performance and can often be avoided.
A high fraction of pipeline slots was utilized by useful work.
While the goal is to make this metric value as big as possible, a high Retiring value for non-vectorized code could prompt you to consider code vectorization. Vectorization enables doing more computations without significantly increasing the number of instructions, thus improving the performance. Note that this metric value may be highlighted due to Microcode Sequencer (MS) issue, so the performance can be improved by avoiding using the MS.