At this point in the play, Brabantio has had to accept that Othello did not use witchcraft or sorcery to win the heart of his daughter, and he has had to accept also that there is nothing he can do about the fact that his daughter Desdemona has married Othello. In the given quotation, Brabantio is warning Othello that Desdemona will deceive him, her husband, just as she has deceived her father. Brabantio is telling Othello that he should keep a careful eye on his new wife.
When he says, "Look to her," Brabantio means that Othello should always carefully watch Desdemona. He is implying that Othello should expect Desdemona to betray him, as she has betrayed her father. It is also important that, in this quotation, Brabantio reminds Othello that he is a "Moor." This reminds Othello that he is a black man and, at least originally, a Muslim, in a white, Christian Venice. In other words, Brabantio is here reminding Othello that in terms of his color and his religion, he is unworthy of Desdemona. This is a persistent idea that Iago also later manipulates to convince Othello that Desdemona's love for him is only a fleeting fancy.
Brabantio then says to Othello, "if thou hast eyes to see." The implication here is that Othello is perhaps not perceptive enough to see Desdemona for who she really is. Upon hearing this Othello may question, albeit only momentarily, whether Desdemona is really as perfect and as angelic as he thinks she is. Othello may also question whether he is perceptive enough. It is also important that the quotation is formed as a rhyming couplet. This makes the sentiment expressed more emphatic and more memorable.