The tool is accessible via a menu in WH XGEN NPC Editor.
It allows viewing an AI recording of a brain you created during some test run and see what went wrong. It is very similar to live-debugging of a tree in XGEN NPC Editor when you are in Game Mode. Replay allows you to record all desired AI entities and then investigate them ex-post. It is also possible to record replays in the release version of the game. However, you do not have access to these exe files so your options with AI Replays are limited.