Auto Mode is the default lesson mode and can be turned on or off in your Settings by clicking on the gear icon.
When Auto Mode is turned on, the highlighted term in the dashboard is automatically selected when you leave a blue word. The first hint is highlighted by default but you can use the up/down arrow keys to highlight the other hints or Known/Ignore buttons if you prefer. Once you click away from that blue word, the highlighted option will be saved as the meaning of your LingQ.
Using the left/right arrow keys is an even more efficient way of using Auto Mode than using your mouse. Use left/right arrows to move from word to word, and up/down arrows to change your selected option in the Reader Dashboard.