Someone who acts as a surrogate takes the place of another person. If a celebrity leaves her seat to use the restroom in the middle of a big Hollywood awards ceremony, a surrogate will take her place until she returns.
Surrogate comes from the Latin word surrogare, which means "to put in another's place," or "to substitute." An uncle might refer to his niece as a surrogate child if they are very close, especially if he doesn't have his own biological children. Since 1978, surrogate has also been used to describe a woman who carries and delivers a baby for another person or couple.