Understanding crest factor of a voltmeter
For a sinusoid, the peak or "crest" is sqrt(2) times higher than the RMS value.  For other waveshapes, it can be much higher.  The crest factor spec is telling you how high the peaks of a signal can be relative to the RMS of the signal for the voltmeter to still accurately measure the RMS.

Many modern RMS voltmeters sample the waveform many times per cycle, and perform the RMS calculation in firmware digitally.  Each reading is squared, these square values low pass filtered, then the square root taken of this low pass filtered result.

The individual readings are linear, and have some fixed resolution and maximum value that can be converted.  To get RMS, samples at the peaks of the waveform still need to be within the measurement range.  However, the average can't be so low relative to the peaks that you get significant quantization noise.  The meter manufacturer has done all these calculations and is telling you how extreme the peaks can be for the meter to still live up to whatever accuracy it is claiming.