There are a couple of themes evident in the poem. The importance of family is one theme and another is aging.
The first stanza conveys both themes.
As if the space between her open arms is reserved for a child, not this body of a man.
The speaker is talking about his mother's hug and how it feels awkward. It conveys to the reader that a fair amount of time has passed between hugs. Perhaps a year. Perhaps more. I think it is a year based on the third stanza's line,
"it is a tune he plays faster each year."
The tune is the noise made by the bottle of wine tapping on the glass as his grandfather tries to pour it. Regardless of exactly how much time has passed, it is clear that the mom hasn't quite gotten used to the idea that her little boy is all grown up. The poem shows the process of aging that way. But it also shows it by mentioning the wrinkles on mom's face, and the coordination decline of the grandfather.
All three stanzas convey to the reader the importance of family to the speaker. There is a closeness that exists among the family members. The speaker knows their habits by memory.
"Dad still soaks himself in the rain."
"Still soaks" emphasizes that it is nothing new to dad. He's always done it, and still does it.
The third stanza really drives home the importance of family. All four of the people are sitting down to a meal together. That was big my family growing up. It's important with my own family now. A meal together is special, and it is time to bond together as a family. The poem conveys that even though the speaker is all grown up, he still finds time to do important meals with his aging family members.