Depending on your plan type and repository's privacy, you will have a number of allotted credits available.
You can build both public and private repositories on the Performance plan. Open-source projects will consume credits. All private projects consume active user seats as normal. Users who are only building on open source repositories will not be counted as an active user for the billing cycle. You may vote on our open feature request to have open source credits for paid plans.
CircleCI provides Organizations 100,000 free credits per week ($2,880 annual value) to use for Linux open-source projects as long as repositories are public. Private repositories will have 2,500 credits/week available, which replenish every Sunday 12:00 UTC