Winnie, the speaker representing the mental part of human existence, is half buried in sand, "up to her bosom" in Act I of the play. She spends the time pulling various items out of a bag, all the while talking to (sort of) Willie, who represents the physical part of human existence. In Act II things have changed -- Winnie is now neck deep in her cone of sand. "Burial" is an oversimple explanation -- to be sure, Beckett is concerned with it ("We are born astride a grave -- the light gleams an instant, and it's night once more."), but here the gradual immersion into "mother earth" also represents Winnie's change in her relationship with Willie, from husband-wife to child-mother. Beckett's play, like so many two-act plays (a modern form challenging the traditional three-act form of development--complication--denouement) is a dramatic imitation of the idea of things changing only slightly, but significantly -- a near-repetition of our youth re-enacted in old age, when our physicality and our
"mental" facultiest become reconciled. Simply put, the sinking into the sand is a device representing Winnie's aging and growing understanding of her dilemma, her facticity. Beckett spent his entire life trying to express this "ineffable" state.