Anything and everything that relates to your project! People who have backed your project have done so because they want to support you and your creative idea - keeping them in the loop will go a long way to show them that you appreciate their support.
Some creators may post an update every day, some may post more sporadically, however it is our expectation that creators will post at least one new update a month once funding is finished. Posting regular updates is the best way to keep backers informed of a project's progress (and from our experience, a better informed backer is a happier backer).
For ideas on what to post an update about, check out these 50 ideas for sending great project updates.
Further reading:
What are project updates?
How do I post a project update?
How do I include images or other media in my project description and updates?