This is as simple as bearing kits get. The ROTOR BB30 Steel Bottom Bracket Bearing set is for mating 30mm thru-axle cranks with BB30 bottom brackets.
BB30 bottom brackets have a 68mm shell width and a 42mm inner diameter. The bearings have a 30mm inner diameter which mates with the 30mm outer diameter of BB30 cranks.
ROTOR makes both the 3D30 and 3D30+ cranks with 30mm thru-axles. This bottom bracket allows you to install those cranks. It also allows for the installation of BB30 cranks from other manufacturers. And BB386 EVO cranks, pretty much only made by FSA.
The kit consists of two clips, two bearings, and two shields. You’ll need a press to install it.