When you are dating two or more people, and feel as though you would like to have a better relationship with only one of them, you slowly spend more time with that person, but until you are sure of your relationship with them you keep the other around just in case things dont work out with the other, and you need your backup instead.
This person is your Insteady
Bob is dating Jill and Jane. He decides to take things more seriously with Jill, but keeps Jane around as his Insteady. He is now "going steady" with Jill, and insteady with Jane.