Reflection is signal reinforcement or cancellation (nulling) due to direct and reflected signals being in or out of phase based on distance between source and target and the length of the reflected path. Enhancement is usually only plus 3 to 9 dB while nulling can be total. Nulls can be -10 to -30 dB, however signal fill from multiple reflections in most indoor environments will generally reduce nulling losses to -6 to -12 dB or so.

Blocking (not in the chart) on the other hand, can be total in some locations. Blocking occurs when an object larger than a half wavelength gets between the source and the target and shadows the target completely from the source. This will be more of a problem at higher frequencies (microwave) than at lower frequencies (LF, HF or VHF).

Skip Interference / Tropo refers to out of area signals which may arrive at levels quite strong compared to the 
desired local signal due to refraction from the earth's ionosphere (HF) or tropospheric ducting (VHF / UHF).
Tropospheric ducting occurs along the boundary between air masses of different temperatures. Consult the
ARRL VHF Manual or RSGB VHF/UHF Manual for more complete descriptions of these propagation modes.