a dog: This phrase is a metaphor (a word picture) for a low-status servant who bows and obeys others. Goliath used it to insult David and treat him as worthless (see 2 Samuel 9:8).
sticks: David had hidden his sling. Goliath could see only David’s staff, so he thought David was coming with a simple weapon.
by his gods: Goliath called on his gods, so David's win would also show God's victory over Dagon, the main god of the Philistines (see 1 Samuel 5:2, 5).