Colorful image colorization

Studying the paper “Colorful image quantization”, I have a question about section 2.1

What is the meaning of

We quantize the ab output space into bins with grid size 10 and keep the Q = 313 values which are in-gamut

I do not know how to quantize the ab color space(ab of CIE LAB) as the paper mentioned. Could anyone explain how to do it or give me some examples? Thanks