The speaker describes noticing a boy in loving detail: the way his eyes light up when he talks about music, the gold the sun brings out in them, his red hair seeming to blaze. From these small observations she realizes she loves him. But the feeling goes one way. He never notices her, or the glances she keeps sneaking at him.
The turn comes in the middle of the poem, when she says she "threw" those glances away. It sounds like she is deliberately letting the feeling go. She decides he doesn't need her, since he seems complete in a world he has built for himself, and she doesn't want to intrude on it. The final line, "after all he's given," is a little puzzling. It may hint that he has given her something, perhaps through his music or his presence, and that she feels she owes him her silence.
The mood is tender and wistful, with a quiet ache. Nothing dramatic happens, which makes the loneliness feel real and understated.