In many internet forums, posting a reply to a thread will make that thread pop to the top of the page. This allows people to see the most active, most recently posted to threads first.
If you want to post a reply to a thread but do not want the thread to be bumped to the top, some forums will allow you to type "sage" into the e-mail field, letting you to post without bumping the thread. This allows minor comments to pass unseen, and also keeps trolls from seeing the thread thrust to the forefront too often.
The word sage comes from a Japanese word, sageru, which means to 'move down'. While in Japanese forums this is in common use, in American forums sage has come to mean something slightly different. On sites like 4chan and 7chan, sage is used like a down vote. Posting sage is an insult, or a vote against whatever was posted above. This probably started because people complaining about a thread did not want to bump it up, and so used sage. After a while, the complaint left out completely, and just the simple word sage gave the vote of disapproval. On some forums, lots of sage comments can also lower a thread's future ability to be bumped up.
This can be confusing to a newbie, as calling someone a sage is, in English, more likely to be a complement to one's wisdom rather than an insult.