Bingo fuel, in a tactical sense, is the fuel at which time you need to terminate the mission and return the base of intended landing with appropriate fuel reserve. It will vary depending on the type of aircraft you're flying, how far you are from your landing base, and the weather at that base. For example, if you are maneuvering in a MOA right near your base, you would only need a little extra gas to get you back to the FAF/initial with the appropriate 11-202 reserves. However, if you're in a MOA that's 50-100 miles away (which is often the case here in England), you'll have to have enough fuel to get home. Then, if the weather at your base is crap (which is often the case here in England), then you have to carry arrive with enough fuel to be able to divert if the weather goes below mins.
Example for the Strike Eagle - AFI 11-202 says that we have to be at initial with 2500 pounds of fuel. If I'm in the MOA 50 miles away and it requires about 2000# of fuel to get home, my bingo is 4500 pounds. Now, if I have an alternate mandating that I arrive at the FAF with, for example, 6900 pounds, my bingo is now 8900 pounds.