It is the maximum rate(in an opamp) at which its output voltage is capable of changing.
The rate of change in output voltage with respect to time is known as slew rate.
It is the response time for an amplifier to slew from one voltage to another voltage in its outputs slew rate explained | ken-gilbert.com
The best idea is to observe the slew rate effect via simulation.
Take, for example, a simple universal amplifier (like LM741) with 100% feedback (unity gain configuration).
Power supply: +/- 10 volts.
Input: Sinusoidal 8...9 volts (amplitude) with a frequency of app 15...20 kHz.
The output will not be sinusoidal due to the limited slew rate.
If you lower the amplitude and/or the frequency the form of the output signal will be again sinusoidal.
You must know slew rate is the maximum rate at which output of op-amp can change. The output will not same as the input in response having some non linear distortions. This phenomena is more effective at higher frequencies.