The noun grub can refer either to a soft, young insect or to hearty food. If your best friend invites you over for some grub, don't worry — she almost certainly means the food and not the bugs.
The grubs your dad picks off of his tomato plant are insects, while the grub you eat in the school cafeteria includes spaghetti and Sloppy Joes. The most interesting thing about this word is how it went from describing a squishy, arguably gross little beetle larva to an informal term for something delicious. Experts guess that it comes from birds gobbling grubs. This slang definition has been around since the 17th century.