What’s the difference between tú and usted in spanish

What does it mean to conjugate a verb? how do you conjugate verbs in spanish?

Mary just sent out invitations to her wedding last week the invitations were so heavy that she put more than just one stamp on each envelope she used 255 stamps and mailed out 85 invitations and see you as an equal amount of stamps on each envelope

Which of the following would fit these sentences? 1. es importante comer de grupos de comidas cada dia2. estoy (with you); los guisantes son saludables3. en una ensalada tomates no hay muchas grasasword bank: 1. la salud, 2. bien, 3. lechuga, 4. comida, 5. que si, 6. todas, 7. prefieres, 8. de acuerdo