I saw pliers and did the same, assumed it’d be for the master link on the chain. But I think he’s reffering to channel lock pliers. (google it, you’ll know)
Changing a cassette is pretty easy. I’d recommend GCN’s “how to” videos on youtube.
Good luck!
What do you do with the pliers?
Cassette pliers, instead of chain whip…

That’s crazy!
pliers - might need to adjust the rear derailleur
You need pliers or a cresent wrench to hold the lockring tool. When removing the cassette you are working two levers, the chainwhip and the wrench/pliers
Alternately you could buy 2 of those park-tool chain-whip tools – the ones I linked to above – as I did by accident; I couldn’t find my chain whip, bought another one and then found the original.
I was about to give the 2nd one away when I realized that the other side of the tool has a slot that is the perfect size to use with the Shimano lockring tool as a wrench. So I use one as the chain whip and the other as the wrench. Easier than using the crescent wrench as it is longer, has more leverage, and doesn’t require and adjustment.
What do you do with the pliers?
Open the beer, sheesh.