In just four lines, the speaker admits to a feeling they badly want to share but can't. The "butterflies" are most likely that fluttery, nervous excitement we get when we're in love or hopelessly attached to someone, though the poem never says so outright. That's part of the point: the speaker can't quite say it, even here.
The first two lines are bold. The speaker wishes they could "scream it to the world," which suggests the feeling is big, bursting, almost too much to hold in. Then the third line pulls the brakes. "The fear in my head" is what gets in the way, and the final line says it "creates a end to my words," as though fear simply cuts the sentence off. The small grammar slip ("a end") even makes the words feel a little broken or rushed.
The mood is a mix of longing and frustration. There's joy in the butterflies, but it's trapped behind silence, leaving the poem feeling tender, shy, and slightly sad.