英-Grab a bite to eat./counting c

3rd. S1

D:Hey man, I'm so hungry.
A:Ya, me too.
D:You wanna grab a bite to eat?
A:Sure, I think there's a fried chicken restaurant nearby.
D:Fried chicken? That's so unhealthy. I thought you were on a diet.
A:I don't care. I just want my fried chicken.

''Grab a bite to eat",  usually it means not to go to a restaurant and wait and have a big dinner (fancy restaurant). It usually means to go to a fast food restaurant, maybe like McDonald's or KFC, and eat quickly.

"On a diet", "lose weight", ''counting calories''. 
They're keeping a track of how many calories they lose and how many calories they take in from eating  when they're working out.