These are objects that have changed during the current session. They could be deleted files, simply anything. When you run a compact and repair it attempts to resolve the issues and if done it deletes them from the sysobejcts file.
For example, and you can try this, create a new table nothing special. Next delete it. Next press Ctrl+Z to restore the table. Ie undelete it. If Access did not store the details of the table int he sysobjects table it would not know how to undelete it. By prefixing it with a tilde ~ it knows that this is a significant file.