How can I use lookup_snv by terminal on Mac?

I try to use lookup_snv function to see how does it work? But I really don’t know the exact input of this function. For example, if I want to lookup_snv (just to do a test :)), I type the lookup_snv through terminal, and then type, and then input the block number 491930 (see from the website), and then it points to input the "A trusted consensus hash, Blockstack transaction ID with a consensus hash, or serial number from a higher block height than block_id (“trust_anchor”): " What’s this? How can I get this information? Thanks a lot.