An assembly language is a low-level way to instruct a computer to carry out a task. Computers do not inherently understand instructions from people. At the most basic level, computers understand instructions in binary language, that is, sequences of zeroes and ones. Binary language or machine language is extremely cumbersome to program in, however. Assembly language was invented as a symbolic representation of the underlying sequences of zeroes and ones.
Suppose we were to program a game which, upon a certain action, awards the player with five points. The computer keeps the score in a certain location; the locations are either a storage place within the CPU, called a "register," or in some space in the memory. The computer understands locations and low-level actions called operations. So if the score is kept at register RA, then it understands the command, 0101 1100 0000 0101, where the first four numbers indicate the operation ADD, the next four indicate the register RA, and the last eight indicate the number 5.