In chemistry terms, a double (pi) bond occurs anytime two atoms bond together using two pairs each of their electrons instead of just one. This phenomenon also occurs during a triple bond, which consists of two pi bonds and one sigma (single) bond. The double bond equivalent is the number of hydrogen you need to convert all of an organic compound's double bonds to single bonds. You can calculate this with an equation if you know the molecule's formula.
Count the number of hydrogen, carbon and nitrogen atoms (if any) in your compound. For the compound C4H2N2, for example, there are four carbon (C), two hydrogen (H) and two nitrogen (N).
Substitute the numbers in Step 1 in the formula DBE = C - (H/2) + (N/2) + 1. For C4H2N2, you see that: DBE = 4 - (2/2) + (2/2) + 1 = 4 - 1 + 1 + 1 = 5.
Check your answer by drawing out your compound and counting the double and triple bonds. For C4H2N2, remember first that nitrogens and carbons must bond using a triple bond, as nitrogen bonds using three pairs of valence electrons. Draw a chain of four carbons with a nitrogen at each end, drawing in the triple bonds. Because you know that each of the end carbons--like all carbons--have four pairs of valence electrons and have already used three to bond to nitrogen, you can conclude that each of these carbons will be single-bonded to the inner carbons. Since there are two hydrogens and only two atoms without full valence shells--the inner carbons--you can conclude that each of these inner carbons will have a hydrogen bonded to it. As they each still have a free pair of electrons, you can also conclude that these two carbons will be double-bonded to one another. Looking at your drawing, you will see one double bond and two triple bonds, for a total of five pi bonds, the same as the answer you got from the equation.