It means if you successfully complete the drug program, they will dismiss the jail time. If you had a choice, and opted for the drug program, I hope you are ready to do it. It's not as easy as it sounds unless you are dedicated to staying clean, which I hope you are. If you are still unsure of the definitions of the sentence, you need to speak to the DA or your Attorney and be VERY clear. You don't want to assume anything with this serious of a matter.