In the scriptural sense, it means to have two allegiances, one to God, one to the world, which is an internal battle that ultimately destroys a person as their is no focus in life. One cannot live correctly if you are a part time believer, and a part time. Can you continuously be a liar, a thief, a murder, adulterer and claim to live a life that strives for righteousness following Godly principles? The short answer is no as those are diametrically opposed, every decision will be compromised, granted if you earnestly stopped living the life of a thief, murderer and so forth, that is completely different. You can't follow two opposite like that without a lot of internal contention, and that will be shown in every aspect of your actions.