Assembly language is fundamentally the local language of your computer. In principle the processor of your machine understands machine code (consisting of ones and zeroes). But with the intention of write such a machine code program, you primarily write it in assembly language and then use an assembler to translate it to machine code.
on the other hand, nothing is vanished when the assembler does its conversion, given that assembly language simply consists of mnemonic codes which are effortless to remember (they are like words in the English language), which take each of the different machine code instructions that the machine is able of executing.
An assembler would translate this set of instructions into a sequence of ones and zeros (that means an executable program) that the machine could recognize.