From the near-essential atla-annotated:
Praying at the Dinner Table
“We are grateful for this delicious food, for happiness and compassion…”
Yes, Tenzin was saying a blessing/prayer at the dinner table. Keep in mind that they live in a quasi monastery. They live on Air TEMPLE Island. Buddhist monks pray before eating.
And no, we never see Aang do this, but then we never really saw him eat at the flashbacks at the Air Temple.
Examples of a Thai monastery day: Click me!
More about the Air Nomads.