Rahab is the prostitute who hid the spies in Jericho.
Israel was approaching the Promised Land after wandering in the desert for 40 years. Joshua (who is now leader after Moses) sends two spies to the city of Jericho. The king of Jericho gets wind of the spies and searches for them, but Rahab, a prostitute, hides the spies under some flax on her roof and later that night, lets them escape over the wall.
Rahab asks the spies for kindness to her family and the spies grant it by telling her to take all her family into her house and tying a red cord at the window.
A little while later, Israel comes and decimates the city of Jericho by marching around it and after the city is a pile of rubble, Rahab is rescued.
If you look at the genealogy of Christ, it appears that one of the Israelites married Rahab and she is in the lineage of the Savior.