LineOfSight (LOS) is the direct path between two points. In WirelessNetworking it is often used to describe the path between two antennas.
Clear LineOfSight means there are no trees, buildings, or other object(s) located in the direct path. Low-E glass (has a slight tint, usually), while seemingly innocuous, is an excellent attenuator (in fact, it is manufactored for this purpose).
Even though you may have a clear LineOfSight, you must account for the FresnelZone of the radio beam.