A public campaign: can be seen by all our creators in their campaign overview. Therefore, they can see your public campaign and sign up for it if they meet your follower requirements.
You should choose the setting when creating the campaign in step 5 under "Publish".
This is shown below:
A hidden campaign: cannot be seen by creators in their campaign overview, it can only be seen by the creators that you actively invite to the campaign. Therefore, be aware that you will not receive any campaign applications for a hidden campaign.
The hidden campaign is usually used by creating it for a specific or exclusive collaboration with one or more creators.