Yiftach (Jephthah) acted impulsively by offering the first thing to greet him as a sacrifice. Unfortunately, that turned out to be his own daughter. Nowhere, however, does it say that he actually sacrificed her, human sacrifice being detestable to G-d. The Radak tells us that Yiftach built his daughter a house, where he sequestered her away from society. The custom referred to, as continues in the next verse, is that each year other girls would come for four days to bewail her fate.