The speaker feels that the louse should go to a pauper's hovel where it belongs and where it will have plenty of companionship with others of its species and that it does not belong on the best apparel of a pretty young woman who is attending church and would be horrified if she realized that the louse was not only spoiling her appearance but creating a bad impression of her hygiene, her home environment, and her social status. This poem is famous because of the last stanza, which reads in the original Scottish dialect
O wad some Power the giftie gie us
To see oursels as ithers see us!
It wad frae mony a blunder free us,
An' foolish notion:
What airs in dress an' gait wad lea'e us,
An' ev'n devotion!
The poor girl who has brought the bug itno church with her is evidently putting on airs in dress and general behavior. Only the speaker knows that she is not what she seems. Burns includes himself and all of us in his moral, and most of us probably well deserve it, at least some of the time.