To repost is to post something again — most often, to share someone else's post from social media onto your own page, so your followers can see it too. It is a very casual, internet-only word; you will not find it in old dictionaries because it only became common with social media. A repost is different from writing your own new post: you are passing along something that was already made, usually with a click of a button rather than any new writing.

轉發、轉貼（社群媒體用語）