A CPU register is a high-speed storage area in the CPU that represents all data prior to processing. Registers inside a CPU are also related to the amount of bits that it has. Bit size is a variable that helps determine a CPU's power and speed.
A 64-bit CPU, for example, has registers that are 64 bits wide. In this case, a 64-bit CPU can use one register to manipulate up to 64 bits of data. Data that needs to be processed by a CPU has to be present in the register, such as two numbers and the result of subtracting one from the other. Data movement through CPU registers are usually not seen by users.