In order to understand writing code in assembly language, you must understand the concept of the accumulator. An accumulator is a temporary memory address used in the CPU of a computer that stores the result of the previous computation. After every computation, the value stored in the accumulator is replaced by a new result.
The main reason that the accumulator exists is so that temporary data can be stored without being committed to permanent memory. This is useful because no space is used on values that may only exist to be used in the following process. Modern general-use computers usually do not have dedicated accumulators because they can gain more functionality from regular memory.