It means that pilot is bidding at 50%. He has moved half way up in seniority in his seat and base. The most senior 737 FO in EWR would be at 1% while the most junior would be at 100%
For example: If there are 500 737 FO's in EWR, and you are number 250 in seniority, you would be at 50%.
PBS uses your base percentage to determine what line to build for you. Although, it doesn't usually work out that way.