The adversary is trying to run malicious code.
Execution consists of techniques that result in adversary-controlled code running on a local or remote system. Techniques that run malicious code are often paired with techniques from all other tactics to achieve broader goals, like exploring a network or stealing data. For example, an adversary might use a remote access tool to run a PowerShell script that does Remote System Discovery.
Most of these techniques do not require a Tool but just access and Native Tools from the Windows Machine itself one of the reasons we want to use Execution on Windows-Signed binaries is to mainly avoid detection or:
Avoid creating new processes/network connectionsAvoid creating anomalous parent/child relationshipsAvoid creating/modifying files/registry entriesAvoid creating memory anomaliesAvoid leaving evidence in log files