When you're training someone or giving them instructions, you can point something out with this phrase:
See how the "ready" light just lit up? That means that it's ready to use.
This phrase is equal to saying "The 'ready' light just lit up. Did you see that?"
Here are more examples:
A: See how it just cut off?
B: Yeah. What happened?
A: You let the clutch out too quickly.
See how the stripes are really dark on this one? That means it's ripe.