Are you serious? (Mostly) Every upvote gives you a point of karma, every downvote takes one from you. Various facilities in HN have a threshold karma requirement, so when you've garnered enough upvotes (and hence karma) these facilities become available to you.
Stories and comments are sorted so that more upvotes push them higher, but they drop over time. Sort order is votes divided some exponential of time, so newer comments/stories appear higher so they have a chance to get noticed, but then drop to the order dictated entirely by votes after a suitable length of time.
All this is in the FAQs, but you said you didn't understand, so I thought I'd provide a high-level summary.
Stories and comments are sorted so that more upvotes push them higher, but they drop over time. Sort order is votes divided some exponential of time, so newer comments/stories appear higher so they have a chance to get noticed, but then drop to the order dictated entirely by votes after a suitable length of time.
All this is in the FAQs, but you said you didn't understand, so I thought I'd provide a high-level summary.