I found the answer:
Trust Level 0 — New
By default, all new users start out at trust level 0, meaning trust has yet to be earned. These are visitors who just created an account, and are still learning the community norms and the way your community works. New users’ abilities are restricted for safety – both theirs and yours.
(We also want to hide any “advanced” functionality from new users to make the UI less confusing for them as they gain more experience.)
Users at trust level 0 cannot …
Send personal messages to other users
“Reply as new topic” via Link button (UI removed)
Post more than 1 image
Post any attachments
Post more than 2 hyperlinks in a post
Have actual links in the ‘about me’ field of their profile (will be silently and temporarily converted to plain text)
Mention more than 2 users in a post
Post more than 3 topics
Post more than 10 replies
Admins can change these limitations by searching for newuser and first_day in site settings.
Trust Level 1 — Basic
At Discourse, we believe reading is the most fundamental and healthy action in any community. If a new user is willing to spend a little time reading, they will quickly be promoted to the first trust level.
Get to trust level 1 by…
Entering at least 5 topics
Reading at least 30 posts
Spend a total of 10 minutes reading posts