An ideal op=amp has an an infinite frequency response This means that no matter how fast the input changes the output will be able to keep up in a real op-amp this is not the case If the input signal changes too fast then the output will not be able to keep up This is defined as slewing and it results in distortion of the output waveform Stated more formally
Or the maximum rate at which the output can change without distorting This can be measured by applying a high frequency square wave signal The frequency of the waveform should be increase until the waveform becomes a triangular wave The slope of the triangular waveform is the slew rate