The "pipe" is an imaginary column/path projected from a dropship to the ground. 5 by 5 has 2 meanings: In this case it means that they are directly in the middle of the "pipe", or rather, in perfect position for decent to the ground. In communications via radio transmissions 5 by 5 is rating 1-5 of signal strength and clarity of the transmission, 1 being the weakest and 5 being perfect. It is the same as saying "loud and clear".
