Literally "I humbly receive." It is a ritualistic expression of gratitude said before every meal to acknowledge the sacrifice of the plants and animals, as well as the work of the cook.
When a protagonist introduces their partner to their mother, hearing the partner say "Okaasan, itadakimasu" signifies that the partner has been accepted into the family. okaasan itadakimasu link
So, the next time you sit down for a meal—whether it is a bowl of instant ramen or a seven-course kaiseki—press your palms together. Think of the person who made it. And say it clearly: Literally "I humbly receive
Additional resources and examples of Japanese table manners and cultural practices: hearing the partner say "Okaasan