Andy
Your "Bid" is the line you bid or were awarded for the month. In other words this is the month's scheduled flying. Say you were awarded a line that paid 75 hours. MOST airlines you are paid what your line pays or what you flew which ever is greater. At the end of the month say you under flew your line and ended up with 73 hours. You would get paid 75 hours. If you happened to over fly then you get paid what you flew, say 77 hours.
Your bid is based on your seniority. If you are number 1 you get what you bid. If you are 100 out of 300 you might get your first choice or 100th choice. The better or more productive lines would normally go more Senior and the bottom pilots would get what was left over or reserve.