Beam tilt is used in radio to aim the main lobe of the vertical plane radiation pattern of an antenna below (or above) the horizontal plane.
The simplest way is mechanical beam tilt, where the antenna is physically mounted in such a manner as to lower the angle of the signal on one side. However, this also raises it on the other side, making it useful in only very limited situations.
More common is electrical beam tilt, where the phasing between antenna elemnts is tweaked to make the signal go down (usually) in all directions. This is extremely useful when the antenna is at a very high point, and the edge of the signal is likely to miss the target (broadcast audience, cellphone users, etc.) entirely.
Occasionally, the two will be used together for odd situations, in order to create greater beam tilt in one direction than the other, mainly to accommodate unusual terrain. Along with null fill, beam tilt is the "focus knob" of radio communications, and together can create almost infinite combinations of 3-D radiation patterns for any situation.