In digital design, register are the basic elements which are used widely. Chisel provides a register , which is collection of D Flip Flops. The register is connected to a clock and the output of the register updates on every rising edge. When an initialization value is provided at the declaration of thr register, it uses a synchronous reset connected to reset signal. A register can be any chisel type that can be represented as a collection of bits.

Below line defines an 8 bit register, initialized with 0 at reset:
val reg = RegInit(0.U(8.W))

An input is connected to the register with the := update operator and the output of the register can be used just with the name in an expression

reg := d
val q = reg

A register can also be connected to its input at the definition:

val nextReg = RegNext(d)

A register can also be initialized during the definition:

val bothReg = RegNext(d, 0.U)