The Mean Anomaly indicates where the satellite was located within its orbit at the specified Epoch.
The Mean Anomaly at any time t, M(t), can be determined by adding the last known Mean Anomaly, Mo, to the orbit's Mean Motion multiplied by the time that has elapsed (t - to):
M(t) = M0 + n (t - t0)
The Mean Anomaly can range anywhere from 0 to 360 degrees.
For the ISS TLE:
"249.9177" = A Mean Anomaly of 249o.9177.
Mean Anomaly Was Last Modified On May 23, 2010