What is "Parse CPU to Parse Elapsed %" ?

Parsing is fully CPU Bound Activity.In ideal case Parse CPU should be equal to Parse Elapsed.If the "Parse Elapsed is more, then that indicates the Parsing activity is waiting on some event(eg Acquiring a Latch).This would give how much wall clock time is elapsed for parsing activity has taken  to complete the Parse request when it takes. 

Total Time for Parsing(Parse Elapsed) = CPU Time Taken to Parse + Wait time .

So

Parse CPU to Parse Elapsed ratio = [Parse CPU/ Parse Elapsed]*100

Low Value for this ratio is an indicator of latching problem. Investigate the Latch Sections in AWR and Statspack report for contention on library cache and shared pool latches.

Ideally Parse Elapsed must be equal to Parse CPU, i.e., only CPU time is used for parsing. In that case the ratio is 100%. If wait time is more then the ratio will be less.