NASA refers to the core stage as "the backbone of the SLS rocket." Hot-fires are dramatic affairs that are meant to simulate the rigor of launch conditions. This event was set to be the final part of the "Green Run" series of tests designed to check out the core stage of SLS before it actually launches from Earth.
Here's NASA's description of what happens during the SLS hot-fire: "On test day, engineers will power up all the core stage systems, load more than 700,000 gallons of cryogenic, or supercold, propellant into the tanks, and fire the rocket's four RS-25 engines at the same time to simulate the stage's operation during launch, generating 1.6 million pounds of thrust."
