what is Atmospheric drag in satellite communication?

 Atmospheric drag
For near-earth satellites, below about 1000 km, the effects of atmospheric drag are significant. Because the drag is greatest at the perigee, the drag acts to reduce the velocity at this point, with the result that the satellite does not reach the same apogee height on successive revolutions.
The result is that the semimajor axis and the eccentricity are both reduced. Drag does not noticeably change the other orbital parameters, including perigee height.  
In the program used for generating the orbital elements given in the NASA bulletins, a pseudo-drag term is generated, which is equal to one-half the rate of change of mean motion.  
An approximate expression for the change of major axis is

where the “0” subscripts denote values at the reference time t0, and n0 is the first derivative of the mean motion. The mean anomaly is also changed, an approximate value for the change being:

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