I got a D in a college class that required a C to pass (Yes I'm an idiot don't rub it in) but...?

D actually gives you the unit; however, you would not be able to get the G.E (General Education) and Pre-requisite to move on. Let's say the course gives you the 'A' requirement, you would not get the 'A' requirement fulfilled because you did not pass the class with a C or better. If that class was required for your major, then you should retake that class because they do not want you to have a D for your major classes. Also, it might impede you from taking higher lvl classes. Let's say you got a D in pre-calculus, you would not be able to move on to calculus. You need a C at least.

Furthermore, it's better to retake the course because you can boost up your gpa if you get a better grade. They will take off that D in your gpa, but it remains in your transcript to show that you failed a class before.

If you need to pass it in order to continue on to another class, you absolutely will need to retake the class (and chances are there'll be an online system enforcing that you got a C in order to continue to the next course).

If this is a required class - for example, for your major - you won't be allowed to graduate unless you retake it and get a C or better. Talk to your advisor about all this.