Hello, Can someone explain me this step that scientist have done.

Then, only bi-allelic SNPs (variant quality score > 200, cumulative depth > 200, individual allele depth R 3) were taken into analysis, and those were further subsetted to cases where an alternative allele was absent in time point 1, to simplify calculations of mutation accumulation rate.

This is from the 2019 published article:

The Human Gut Virome Is Highly Diverse, Stable, and Individual Specific

Thanks, Vishal