It's a term-of-art that means to use the tools that are already available on the target machine. So rather than shipping a custom binary/shellcode/etc which ex

It's a term-of-art that means to use the tools that are already available on the target machine. So rather than shipping a custom binary/shellcode/etc which exfiltrates data or whatever, you string together existing powershell/unix/etc commands to do so. It's effective because it's hard to distinguish these from legitimate processes.