What does this sentence mean?
"Too many of us have forgotten that meals are usually eaten sitting down at a table"
Does it mean, we forgot, the fact that meals are usually eaten when we are sitting down.
or does it mean, as we sit down, we forgot the fact that meals are usually eaten.

Question

Answer

Many of us live in an urban environment where we're always rushing from one place to the next with little time to sit down and eat a proper meal. College students grab snack foods from campus vending machines because they don't have time to eat between classes. After work, you might just have enough time to shower and get dressed to catch a movie with friends: "Sorry, Mom, I told Patty and David that I would meet them at 7:00 and I'm already la-a-aaate!

Answer

It means that we have forgotten that meals are eaten while sitting down at a table.  I think it is referring to the fact that people are always in a rush these days and do not sit down to a meal like they used to. 
Hope that helps!

Answer

Neither.

"Sitting down at a table" describes the manner of the eating.  The sentence is saying that meals should properly be eaten with everyone sitting at the same table while they eat, but we have forgotten that. Instead, we eat sitting in front of the TV, at the computer, or on the run.