Constipated?

So... For two days now (or one, since yesterday and now today as well) I've not been able to poop. I can tell that I have to, because I can feel it, but it won't come out? Is this what constipation is? If so, what is the reason for is and what can I do to be able to poop? I'm worried.
Also: please don't call me stupid or something for not knowing. I'm 16, and this has never happened to me before. I know that being constipated means that you can't poop, but I don't know what it means by that so I don't know if this is what it is or if constipated just means that you don't have to poop for a long time. Anyway, thanks in advance for any helpful comments