"Love is not all, it is not meat nor drink"

[cw: food, death, suicide]

Someone asked

What is the message of Sonnet 30 by Edna St. Vincent Millay?

This is a Shakespearean sonnet. Partly that means it has a particular rhyme scheme, but more importantly it means its meaning has a particular pattern:

• Something. Traditionally this part is eight lines long, as it is in this sonnet.
• Break, called a "volta".
• On the other hand, something else.
• Resolution: tie the two somethings together.

Let's go through it together:

Love is not all

There are lots of things in life other than love. She is presumably referring to romantic love here.

it is not meat nor drink
Nor slumber nor a roof against the rain;
Nor yet a floating spar to men that sink
And rise and sink and rise and sink again;

Here are some important things in life, other than love:  food, drink, sleep, shelter, and a lifebelt if you're drowning.

Love can not fill the thickened lung with breath,
Nor clean the blood, nor set the fractured bone;

Also, love can't provide healthcare. Love can't save your life. But...

Yet many a man is making friends with death
Even as I speak, for lack of love alone.

...but the odd thing is, people often kill themselves because someone doesn't love them.  So love can't save your life, but it seems that lack of love can kill you.

Here's the volta, so now we’re talking about something else. The previous part was talking about love in a theoretical, abstract way. The next bit is addressed to a particular lover.

It well may be that in a difficult hour,
Pinned down by pain and moaning for release,
Or nagged by want past resolution's power,
I might be driven to sell your love for peace,
Or trade the memory of this night for food.

Lots of things might go wrong in the future. When they do, you might want to give away something you have, if it would make things better again. Maybe if things got bad enough, Millay might be tempted to throw away her lover's love if it would fix the situation, or even to lose the memory of one night spent with her lover. "Trade the memory of this night" might also mean telling someone the (previously secret) story of what happened that night, in order to get food when she was hungry.

Note that she says "this night", so she's saying this to her lover as part of that night which was so secret and special.

Resolution:

It well may be. I do not think I would.

"It may well be" introduced the previous section. So, maybe she would give all that up in order to save her life. As she said at the beginning, there's more to life than just love. But she thinks it's unlikely, because her lover and their secrets are so very important to her.