Are you supposed to wait to brush your teeth after all meals?

No: You may brush your teeth after each meal. Use a soft toothbrush.

Yes wait 20 minutes: When you are eating foods that are acidic the ph of your mouth will drop and your whole mouth becomes acidic. Your teeth will dissolve when the ph is below 5.5. Your saliva buffers the acids and get the ph back to a normal and safe level. If you brush your teeth when they are at their weakest, you can erode the enamel. So wait at least 20 minutes before brushing. Fruits and veggies are acidic.

Technically speaking: After eating or drinking certain very acidic foods or liquids, it might be better to wait 20-30 minutes before brushing. One way to get around this is to rinse your mouth with tap water after eating or drinking (this will help reduce the acidity). I would rather my patients brush and floss after every meal than avoid doing that because waiting was inconvenient and they skipped it altogether.