Meanly, in the nineteenth-century, meant to live an ungenerous, miserly, pinched life. It was the opposite of living fully and generously.
When he says "Still, we live meanly, like ants," Thoreau means we have become so focused on working and accumulating that we live narrow, ungenerous lives.
According to Thoreau, what makes our lives mean or narrow is that we allow too many details to accumulate—most of them worthless. We lose the essential meaning of life in an endless pile of work and social details and perceived needs, which divert us from the rich central core of life. Just as a miser accumulates piles of money he can never enjoy, because he never spends them, so we accumulate work which should free us after a time for the good life, but instead becomes an end in itself.
Soon after asserting that most people live meanly like ants, toiling away but not thinking or really living, Thoreau offers his solution to the problem:
Our life is frittered away by detail. An honest man has hardly need to count more than his ten fingers, or in extreme cases he may add his ten toes, and lump the rest.
Simplicity, simplicity, simplicity!
Less is more, say Thoreau.
As the above answer explains the location of this quote well, I will move right into more analysis on it. Thoreau's use of the simile "Still we live meanly, like ants" also suggests the unyielding, unthinking nature of the life of a human. To Thoreau, our daily lives are "frittered away by detail," fraught with regularity as we simply go through the complex series of motions that have been deemed necessary to our survival. What Thoreau wants us to realize is that while we believe this everyday routine to be life, it is rather a shadow of a life—that real life exists outside of the mindless, complex cycles that we adhere to. To Thoreau, an ant lives to simply work. The work is never done, and there will always be more work to do. Thoreau sees the existence of most humans to be very similar. Humans amass material possessions and obligations much in the same way that ants amass food. However, food is necessary to an ant's survival, whereas material goods and obligations only weigh humans down and make their burdens even worse. For Thoreau, a good life, an authentic life, is a simplified life.
This question refers to paragraphs 16 and 17 in the chapter “Where I Lived, and What I Lived For” in Walden. Thoreau first explains in paragraph 16 why he moved to the shoreline of Walden Pond:
I went to the woods because I wished to live deliberately, to front only the essential facts of life, and see if I could not learn what it had to teach, and not, when I came to die, discover that I had not lived. I did not wish to live what was not life, living is so dear; nor did I wish to practice resignation, unless it was quite necessary. I wanted to live deep and suck out all the marrow of life …
He wanted to figure out how he could live life as fully as possible. By contrast, he saw that most people lived only superficially. They skimmed along the surface of life and went along with what everyone else thought and did, without doing too much thinking on their own. This is why he wrote later, “The mass of men lead lives of quiet desperation.”
Paragraph 17 begins with the phrase: “Still, we live meanly, like ants.” Here Thoreau uses “meanly” in a way that is unusual for us today. Look in a dictionary, and you’ll find that the word “mean” can be used as an adjective meaning “low in quality,” “poor,” and “inferior.” He continues by identifying what part of the problem is: “Our life is frittered away by detail. … Simplicity, simplicity, simplicity! I say, let your affairs be as two or three, and not a hundred or a thousand.” And so on. Thoreau suggests strongly that simplifying one’s life is the solution to living deliberately and not meanly.