Is the delayed Entry program a good idea?

The delayed entry program isn't going to hurt your military career in any way. it's just the delay of your entry into the military whether it be, waiting for a waiver to be approved or waiting to ship out to basic training.

Could you tell me what the delayed entry program is.. and why it is needed?

The delayed entry program is nothing except a way for you to "pre-enlist" in the military. You will still have to raise your right hand and take the oath of enlistment right before you ship for basic training. You also have the ability to complete some training on the weekends at a reserve unit in order to come in one rank higher (max E-4) than you would have otherwise. Its main benefit to you is that any enlistment options that you are looking at right now may be gone later, so if you use the DEP you get the benefits that are being offered right now.