Two short couplets, and they hold a dark thought inside a party scene. The speaker pictures a group of people enjoying themselves, "our revels," while a clock hangs over them. But the clock isn't just a clock. It's an angel, and a leashed one, which suggests that death is being held back for now, though not forever.
The second couplet turns personal. "I give it a while to betray us" has a dry, almost casual tone, as if the speaker has accepted that time will let everyone down in the end. The angel will "tally all our time," counting up what we've used, perhaps like an accountant closing the books.
The mood is uneasy but calm. There's no panic here, just a quiet awareness that the fun is borrowed. The speaker seems to enjoy the revels while knowing the leash won't hold, and that knowledge gives the poem its chill.