For every point of strength below the requirement, you reduce your bonus from dexterity by 1 and take a -1 penalty to all physical actions. You also reduce your move speed by 5 for each point.
Shields have no dexterity cap, but some of them reduce your effective dexterity for armor. This is represented as a dexterity penalty to your armor class only if your total is equal or lower than the armor's cap you are wearing. For example if you have a Dexterity of +4, and are wearing a chain shirt, and also wielding a heavy steel shield (-1 penalty) Your Block Bonus would only be 20. 3 from the armor bonus of the chain shirt, 3 from the dexterity bonus, and 3 from your shield. If you had a dexterity of 5 instead, your armor class would be 21, as the heavy steel shield would not decrease your dexterity to below the cap.