Empire is “a pure PowerShell post-exploitation agent built on cryptologically-secure communications and flexible architecture. Empire implements the ability to run PowerShell agents without needing powershell.exe, rapidly deployable post-exploitation modules ranging from key loggers to Mimikatz, and adaptable communications to evade network detection, all wrapped up in a usability-focused framework.”

Basically, Empire is a tool that is similar to Metasploit but specific to PowerShell. It allows you to run PowerShell scripts in memory and make a connection back to your machine.