What is 'bump'?

I'm fairly new to TSR and I have seen people writing 'bump' on threads. What does this mean/what is the purpose of it?

Thaaanks!

Others already answered your question but forgot to mention that its against site rules If you bump threads, either your own or others, you could receive an alert or warning for spamming.

Thats *******s. Its only against the rules if you do so unreasonably. For example deliberately seeking out old crap threads and bumping them up.

You're telling a moderator that you know the rules better than they do?

Posting 'bump' just to move your thread onto the front page is treated as spam, first and foremost because it is inconsiderate to other forum users. If its a busy forum, a new thread might move onto the second page within a few hours. But everybody else's thread is just as deserving of being viewed as yours is, so to bump your own thread instantly pushes another onto the second page. By and large threads don't receive replies only when the wider community doesn't know or doesn't want to answer a thread.
Secondly, everywhere other than chat, posting 'bump' is spam because it doesn't contribute to the thread topic.

Oh. My statement was based on the fact that this thread itself was bumped without moderator action being taken.

Also what happens if someone "bumps" the thread and then deletes their bump. Does the thread go back to where it was? I cant be bothered to test this.

You're telling a moderator that you know the rules better than they do?

Posting 'bump' just to move your thread onto the front page is treated as spam, first and foremost because it is inconsiderate to other forum users. If its a busy forum, a new thread might move onto the second page within a few hours. But everybody else's thread is just as deserving of being viewed as yours is, so to bump your own thread instantly pushes another onto the second page. By and large threads don't receive replies only when the wider community doesn't know or doesn't want to answer a thread. Bumping a thread also irritates forum regulars because it is very demanding and pushy, and people don't like to help or respond when the thread starter treats it like an obligation.
Secondly, everywhere other than chat, posting 'bump' is spam because it doesn't contribute to the thread topic.

Keep all your posts relevant and constructive.
...
2. Sometimes people want to 'bump' a thread (ie, post in it again so it returns to the top of the chronological thread list). That’s fine once, if your thread hasn’t had a response after a significant amount of time. But please don’t bump a thread multiple times or without allowing appropriate time for others to read the thread and reply. How much time you should wait for a reply depends on the forum - in busier forums, it becomes appropriate to bump a thread sooner than in quieter ones. As a rough guide, please give it a few hours and don’t bump a thread while it’s still near the top of the most recently updated threads in its forum.

I have been told in the response to a post report (not in chat) that bumping is allowed, as long as it is after a reasonable time period, and that the moderation team prefer it if the OP deletes the bumps if/when the thread is replied to. Is this a change in policy then, and do you know when the policy changed?