I called him garrulous, but then, I did not know That all his words were but sandbags With which he built a dam Around himself.
But now, grown old, I’ve learned most men Who talk too much, Like me, are vainly building dams Against the sea Of loneliness.
Looking back, the speaker remembers once calling someone garrulous, a man who talked too much. At the time it was a simple, slightly dismissive judgment. Only later does the speaker see what the talking was really for. The man's endless words were sandbags, stacked up to wall himself in against something he couldn't face.
The second stanza turns the realization inward. Now old, the speaker admits that most people who talk too much are doing just what that man did, and the phrase "like me" gives the game away: the speaker has been a talker too, building the same wall without quite admitting it. The sea of loneliness is vast, and the word "vainly" tells us the dam will never hold.
The mood is quiet and rueful, a little tender. There's no bitterness here, just the gentle sadness of someone who now recognizes in an old irritation a mirror of himself, and who forgives the talker, and perhaps himself, a bit.