What is the mixed numerology in OFDM In order to meet the user requirements in the next
generation radio access technology, the ongoing standardization within 3rd  Generation  Partnership  Project  (3GPP)  offers flexibility in terms of employing mixed numerology.

I have read many papers about it, such that: This link.

I think it's a new technique but it's difficult for me to understand it well. It's like to use different carrier spacing instead of one fixed of 15 KHz in LTE, but that's not sure. 

Could you please explain what is that mean in an easy way, and what's the benefit of it?
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.