000636 : Skipping feature <value> because a negative distance of <value> resulted in no geometry.
When buffering a polygon feature class, the buffer distance was a negative value that was large enough, in relation to the size of the input feature, to result in the buffer collapsing to nothing (no area). The resulting empty geometry was not written to the output. This warning can also be generated when buffering a line feature class and specifying flat ends. This typically occurs when line features are very short in relation to the buffer distance.
This is an informative message. No further action is necessary.