In just four lines, the speaker makes a strange bargain: to have visions, you have to let your past be altered. Seeing the future, or seeing something beyond the ordinary, isn't free. It costs you the stability of what has already happened, and the poem states this calmly, as if it were a fact of life.
The second couplet turns domestic and a little unsettling. "One long look under the bed" evokes childhood fears of what hides in the dark, yet here the thing you find "will be true tomorrow." The idea seems to be that looking hard at something makes it real, or that what you imagine or glimpse today becomes tomorrow's truth. The speaker doesn't say what lies under the bed, and that gap is where the unease comes from.
The mood is quiet, eerie, and slightly dizzying. Time feels unreliable: the past can be revised, the future can be pulled forward, and the speaker seems both fascinated by this and wary of it.