Here's a drastically oversimplified explanation of slew rate.
Amplifiers with multiple stages and a lot of feedback, like op amps but also like some other circuits, are often unstable -- they'll break into oscillation. To stop the oscillation, compensation capacitors are placed within the amplifier circuit to roll off the gain beginning at a particular frequency.
When the amplifier is hit with a signal that has a very steep rise or fall time (a square wave if you're testing, a fast transient if you're using), an active device has to charge or discharge that compensation capacitor. (I'll call that active device a transistor, although some tubed amplifiers behave in the same way.) To charge or discharge it quickly, the transistor has to source or sink a good deal of current. If the amount of current needed is greater than the transistor can provide (or, in the other direction, the transistor runs into zero) the transistor will go into current clipping. Instead of following the fast-changing signal, the amplifier's output will ramp upward or downward in a straight line whose slope is determined by the size of the capacitor and the current coming from the transistor.
That straight-line ramping upward or downward is called "slewing", and the rate at which it ramps up or down is the "slew rate". During slewing, the amplifier is essentially operating without feedback and produces gross distortion. (Some writers call the rise-time or fall-time of actual signals the slew rate, but this is a misnomer unless the amplifier is actually slewing. If it's just going up and down without that transistor-capacitor overload, it's not slewing, so the rise- and fall-times aren't slew rates.)
Ideally, an amplifier should be operated so that it never slews. One way is to bandlimit the signal before it hits the amplifier, so the amplifier never sees those fast-rise-time signal slopes. Another is to self-bandlimit the amplifier by putting a capacitor in parallel with the feedback resistor.
It should be noted that while the above mechanism is by far the most common source of slewing, it can also happen when the amplifier is asked to drive a capacitative load like a long cable or, in the case of a power amp, an electrostatic loudspeaker. In those cases the output devices reach their current limits while trying to charge the capacitance rather than an internal transistor, but the effect is the same, and bad.
Peace,
Paul
![Soldering Iron Kit, 120W LED Digital Advanced Solder Iron Soldering Gun kit, 110V Welding Tools, Smart Temperature Control [356℉-932℉], Extra 5pcs Tips, Auto Sleep, Temp Calibration, Orange](https://m.media-amazon.com/images/I/51sFKu9SdeL._SL500_.jpg)
