Throwback Thursday is the name of a weekly post theme that social media users participate in as part of a very general "throwback" activity for posting content.
On Thursdays, people usually post photos on sites like Instagram, Twitter, Tumblr or Facebook that are from the past. Photos can be from years ago or from just a few days ago. There aren't really any limitations, and it really just gives people an excuse to post more.
Throwback Thursday is very popular on Instagram, and users often tag their photos with a variety of hashtags like #TBT, #ThrowbackThursday or just simply #Throwback. Adding these hashtags can help get photos more exposure from a wider audience of people who are searching through those tags.
The trend is quickly catching on to other social networks, and some even expand content to include songs and videos from the past as well.