Pipe light, which is a special term in a snubbing operation, describes the condition of the well which well head pressure and buoyancy acting upward against a cross sectional area of pipe is greater than weight of the string. If the light pipe condition is happened and the string is not controlled by the snubbing unit, the string will be pushed out of the well.
You may see this VDO showing the pipe light condition. The string of tubular (I am not sure if it is a drill pipe or a tubing) was pushed out by the wellbore pressure. This is the great example of pipe light condition.
After knowing the concept of the light pipe, we will apply it to determine how many feet of tubular required snubbing into the well before the weight will overcome the surface pressure plus buoyancy force.