There are two types of programs you can hack on: Private and Public programs.
Private programs are programs that aren't published to the public. This means that you can only submit vulnerabilities to these programs when you receive specific invitations to hack on them. All vulnerability reports for these programs remain confidential, and you can't explicitly talk about the vulnerabilities you find.
As private programs limit the number of hackers invited to the program, the number of report submissions is also limited to enable the program to get the hang of receiving and triaging vulnerability reports. All programs begin as private, but as they become more proficient in handling reports, they can choose to go public if desired.
The advantage of hacking on a private program versus a public one is that the pool of hackers finding vulnerabilities is much smaller, giving you an advantage to find and be rewarded for vulnerabilities.
When programs become public, they open themselves up to report submissions from the entire hacker community. This means that all hackers on HackerOne are given rights to hack the program. If you're a hacker who's just getting started on HackerOne and hasn't received any private invitations yet, hacking on public programs is a great way to build up your hacker profile of reputation, signal, and impact.