What does the Campaign Member 1000 error status mean?

When it comes to the Status field, Campaign Members act differently than most other object/fields in Salesforce. They have a picklist, Status, where you record the campaign member's status when creating a Campaign Member. It defaults to Sent and Responded, but you can add more options by adding custom Campaign Member Status records to the Campaign. This updates the picklist values available on the Status field on Campaign Member as well. There is a limit of 1,000 picklist values on any picklist field, so you can't add more than 998 custom statuses. 

That Campaign Member Status (CampaignMemberStatus) object is a child of Campaign. When you are setting up a Campaign, each will have its own Campaign Member Status child records. They aren't shared among campaigns, even if they represent the same status value, like "Sent" or "Attended".

So if you use a status if 'Attended' in 3 campaigns, it generates 3 Campaign Member Status records, one per campaign and adds "Attended" to the Status picklist values on the Campaign Member. If you create 500 campaigns, each with 5 different available Campaign Member Statuses (e.g., Invited, RSVP Yes, RSVP No, Registered, Canceled), it will create 2,500 Campaign Member Status records.

Note: Other than the 1,000 picklist values limit, Salesforce also has a limit of the number of those Campaign Member Status Records you can create. This can become an issue if you have hundreds of campaigns, you may hit that limit. Unfortunately, Salesforce is not clear on this limit, but you will receive an error in Maintenance Logs when trying to add a new campaign via the integration.

https://developer.salesforce.com/docs/atlas.en-us.object_reference.meta/object_reference/sforce_api_objects_campaignmemberstatus.htm