What is discharge????

im 17 years old and i started my period when I was 14. what is discharge? everyone seems to talk about it but i have no idea what it is? people say it happens for acouple years before you get your first period i never got that??? some say it happens a week or two before your next period well that doesnt happen to me either? some also say it happens after you have a period well i also dont get it? im sure i dont get it i hear it can be yellow or clear or white but its thick, i have never seen anything like that come out of my vagina during anytime. so is that normal???