If you are referring to the famous and beautiful poem 'Prayer For My Daughter' by William Butler Yeats, then that line refers to his wish that his daughter be not so beautiful that she attracts either harm to herself or to another - for example, a man that may later fall in love with her when she becomes a woman. He reminds us of another beauty - a goddess - who is said to have been born of the sea as Aphrodite. William Butler Yeats may be thinking here of his own bitterly sad experiences of an unrequited love for the love of his life - Maud Gonne. Maud was very pretty, lively and popular - but not just with Yeats. She had very many admirers and would choose none of them! She wanted to concentrate on her own projects, one being politics and the struggle for the independence of Ireland from England. Yeats may be hoping his own baby daughter wont grow up to be such a firebrand!
In this poem, the lines that you are citing refer to the Greek and Roman goddess Venus (that was her Roman name) or Aphrodite. She is said to have been born out of the spray of the sea. Because she did not have a father, she could pick her own husband. However, she ended up picking Vulcan (Roman name) to be her husband.
So, in the poem, the father is asking for all these blessings for his daughter. One of them is that he wants her to be beautiful, but not so beautiful that she makes mistakes like that of Venus.