How do you explain depression?

I've had severe clinical depression for many months. When my parents found out that I was having feelings of hopelessness, they told me to talk to them. I've never had a close relationship with them. In fact it's always been a distant, only speak when I have to kinda thing. The other day I tried to help them understand how I was feeling, and I said stuff like "I feel like dying" they wanted to know why?!?! how am I supposed to explain something like THAT!!