Its literal translation in English is infant of grace.
Infanta refers to the daughter of a Spanish or Portuguese king. Originally (in the Middle Ages), the style of Infante was reserved for the sons and daughters of the king. It was similar to the contemporary style of "filius regis" in France and elsewhere.
Starting in the 18th century (although perhaps before), kings and queens have granted the title of Infante by royal decree (Infantes de gracia).
This second category of Infante, as recognized by the decree of 29 May 1922, art. 1, is that of Infante de gracia o por concesión Real. Although the texts of the decrees do not seem to make any finer distinction, one can nevertheless classify these grants in a few categories.
For more info, see The style of Infante / Infanta de España from Heraldica.org.