I see all the time on posts, do not necropost. But then I asked myself, Why is necroposting bad in the first place? One time I just posted a reply then someone said, no necroposting and I got very confused. Help me, scratch team.

I haven't seen necroposting in Community guidelines.

If the most recent post is more then a a month old, it usually means that the original creator of the topic is not interested/active/forgot. So even if your post is constructive, it's not very helpful to that person, because the topic is dead.
So you're responding to a month-old topic, and by doing so, bring a dead topic to the top of the discussion page.

Can you post things on your things or is it still necroposting even if it was you who made the topic?

Scratchfangs wrote:

Can you post things on your things or is it still necroposting even if it was you who made the topic?

Yes, but must be related to the og conversation

Ok, thank you for the tip. I am not so confused anymore.

Also, there are a few exceptions to necroposting that @ThatOneWeirdDude didn't mention.

You may necropost if:

• It's in Things I'm Making and Creating (I believe), Suggestions, Things I'm Reading and Playing, and a couple other forums (that I'm unaware of) if you still pertain to the topic.
  
• If you're constructively replying to a necropost letting the necroposter know that posting on old topics isn't allowed.
  
• If you figure out some long dead yet still somehow relevant issue (because someone else may come across the same problem).
  

These's aren't set in stone – depending on the situation they can get bent.