What is the mixed numerology in OFDM
In general, it means several waveforms will coexist in 5G ecosystem.

In the context of LTE-based 5G (up to now), several subcarrier spacings (hence CP) coexist. The first and foremost motivation for this is to support not only mobile broadband but also other use-cases such as mMTC and URLLC.

Some of benefits of mixed numerology are flexible scheduling and better spectral efficiency.

Anyway, I do agree to other comments that the question is a little too broad.