What is the meaning of "building DAG of jobs"?

I’m new to NextStrain and was reading the tutorial document. https://docs.nextstrain.org/en/latest/tutorials/SARS-CoV-2/steps/setup.html So, After running the command

snakemake --cores 4 --profile ./my_profiles/getting_started
I got the following

Building DAG of jobs...

Updating job 22 (aggregate_alignments).

Nothing to be done.

Complete log: /Users/amanda/ncov/.snakemake/log/2020-11-19T091330.453646.snakemake.log

So, what is the meaning of “Building DAG of jobs…”? Moreover, I understand that this NextStrain has two components, one is “Augur” and the other one is “Auspice”. So, here, before this command, why do we need to explicitly install auspice using

“npm install --global auspice”?

I figured that once the NextStrain environment is created, then both the programs “Augur” and “Auspice” are installed right away.