Assembly (Abbreviated asm) refers to the low level programming language that most modern languages get translated to. When you compile a program it goes through an assembler that translates code into Assembly. This is just a form of human readable code that the machine translates into machine code.
Assembly is one of the most barebones languages as it is a 1:1 translation of what the CPU will interpret including registers, instructions, and other resources. Some good resources on learning more about assembly can be found here .
X86_64 X86_64 is the instruction set that most modern computers run on. A good x86_64 assembly cheat sheet can be found here