Reddit glossary
What is Megathread on Reddit?
A megathread is one big pinned thread that collects all discussion on a topic, like a weekly self-promotion thread.
Mods use them to keep the main feed tidy. You will see things like Weekly Q&A, Show Your Project or Monthly Recommendations.
They are often pinned to the top of the community and replaced on a schedule.
If you are promoting something
If a community has a thread for sharing projects, that is usually the one place your own promotion is welcome. Use it, and read the format the mods ask for.