Albert Barnes on John 1:27

Whose shoe's latchet. Mt 3:11. The latchet of sandals was the string or thong by which they were fastened to the feet. To unloose them was the office of a servant, and John means, therefore, that he was unworthy to perform the lowest office for the Messiah. This was remarkable humility. John was well known; he was highly honoured; thousands came to hear him. Jesus was at that time unknown; but John says that he was unworthy to perform the humblest office for Jesus. So we all should be willing to lay all that we have at the feet of Christ, and feel that we are unworthy to be his lowest servants.