Assembly language is a low-level programming language whose instructions map almost 1-to-1 to a computer's machine code.
When analyzing an executable whose source code is unknown (e.g. the payload of an attack), the analyst can use a tool called a disassembler. A disassembler translates the machine code instructions (the numbers that form the unknown code) to assembly language, helping the analyst to better understand what the code is doing.
Below is a simple example of what assembly code looks like. Although assembly code looks quite cryptic, all the words are names or abbreviations. The example is for the x86 family of processors.
CLC MOV AX, 5 ADD AX, [SI] MOV [SI], AX