An assembler is a type of software tool that translates assembly language, a low-level programming language that is closely related to machine code, into executable machine code that a computer's processor can understand. Assembly language uses mnemonics and symbols to represent machine-level instructions, making it more human-readable than pure binary code. Assemblers play a crucial role in systems programming, enabling programmers to write efficient code that interacts directly with hardware. Overall, they are essential for tasks requiring high performance and precise control over system resources.