“Itadakimasu” is the polite (-masu) non-past form of “itadaku,” a humble verb for “to receive.” The nuance is as if you are raising something above your head in a humble gesture as you receive it. (As a noun, “itadaku” means “peak/ summit”; the head is the summit of the human body.)

So, “itadakimasu” comes off as “I humbly receive this ___” (with the ___ left unsaid, as it is clear by context, and would be either food or drink, or both).

So, “I humbly receive this meal.”

While some liken it to “saying Grace,” that is thanking God. Itadakimasu is thanking the host.