A reservoir pump includes a chamber that you pressurize with the pump to release a rapid, high pressure flow of compressed air to seat your tubeless tire. It's a useful tool if you're frequently setting up tubeless, although most tubeless tires will seat okay on most rims with a standard track pump.
Tubeless tire tech has got a lot more precise nowadays and with a bit of prep like soaping the tire beads before trying to inflate the tire you'll likely find that you'll get by with a standard floor pump.
Having said that, you'll still find pesky wheel-tire combos that just refuse to seat. Whether it's worth having a reservoir pump just in case or whether if this happens you'll just take your wheel to the bike store is a personal decision.