The speaker is performing, and the lights are so bright that the audience is almost impossible to make out. Somewhere out there, they think, someone was looking at them with a dreamy, longing gaze. They can't actually confirm it, and they admit as much in the first line.
What makes the poem charming is the second line, which begins with "But." The speaker knows the evidence is weak and chooses to believe anyway. "I'm still going to pretend" is honest about the self-deception, and that honesty gives the moment a gentle, slightly sheepish humor. The wording is a little breathless too, with "dreamingly & longingly" piling up as if the speaker can't resist making the fantasy bigger.
The mood is wistful and hopeful, tinged with vulnerability. It captures how a performer, or anyone who longs to be noticed, can turn uncertainty into a small private comfort.