Record starts a new file in the output folder. The button changes to Stop. The status box shows how long the recording has been running, and the file name.
Stop finishes the file so other programs can play it. If the Metrics page is already open, its chart loads the log from that recording.
Replay opens the last file with the player Windows uses for that kind of file. If you have not recorded yet, the status line says so.
If a hotkey is already used by another program, the status line says so. NitroWarp still records from the button and the tray menu.
On the Video page, Stay on top keeps the NitroWarp window above other windows. Turn it on when you still need to reach Record and Stop while you watch something else. The choice is remembered.
The close button hides NitroWarp. A recording that is already running keeps going. The icon stays in the tray, including the overflow area where Windows hides icons.