I have no idea what flag weight stands for.
Which is better, having low flag weight or high flag weight? Could you elaborate on it?
In short, flag weight is a measure of how reliably you flag content. High flag weight is good. It can range between 0 and 750. It increases if your flags are positively reviewed. The more flag weight you have, the more you can flag, and the higher the priority of your flags is. High flag weight is rewarded by a Deputy (500) or a Marshal badge (749).
Flagging posts is a privilege that you gain once you have 15 reputation points; so this is a way of helping the community that you can use pretty early on. There's more information on the corresponding flag privilege info page, the faq entry "flagging", and the extensive answer on meta.stackoverflow.com: What is flag weight?