It means the amount of times a user has received Most Helpful out of the total number of opinions the user has given (but I think it's only based on the questions that actually have a MH opinion selected).
Using myself as an example, I have a Most Helpful percentage of 24%. That means 24% of the total number of answers I have given were chosen for MH. To put that in perspective, out the 4,138 opinions I have given on questions that have a MH selected, 980 of mine were chosen the MH.
Most Helpful opinion can be awarded on each question, either by the Asker or the system after 30 days. Your Most Helpful Percentage is - how many Most Helpful opinions out of opinions on questions with a MHO.
MHO = Most Helpful Opinion Its the percent of most helpful opinions youve received...