The speaker is an American tourist wandering Amsterdam's Red Light district, and the poem opens by poking fun at that crowd: people full of slogans, commercials and Hollywood. Then the speaker admits to being one of them, though they insist they're looking for something more.
From there the poem turns inward. The speaker used to believe answers came from within, but now suspects that the "within" is built entirely from outside things. The image of the self as a bucket makes the point: it can hold the sun, moon and stars, but only whatever gets poured in. Experiences, guts, bullshit, even "selves" all end up not really theirs.
The mood is restless and a little wry, tired but still searching. The ending is funny and unsettling at once. The speaker feels they've missed something, and then guesses it might be "everything."