Auto Crosspost is a Webhooks v2 feature that allows Streamcord to automatically publish notification messages in your announcement channel. Members in your server can 'follow' the announcements channel in order to be notified whenever you go live, instead of setting up a Streamcord notification individually.
In order for this feature to work, your server needs to be a Community Server. Then, you need to create an announcement channel, and set up Streamcord to post in that channel. Then, check the "Automatically crosspost messages" option when adding a notification.