Here's what that means.
Your dashboard lists the beta subjects that you're supporting, i.e. that you've contributed to at some point in time by posting an answer or by editing one.
There are currently
Your dashboard shows the beta subjects that you have contributed to since they were promoted to beta status.
As you can see, you posted or edited answers in US History and World History. Now, when one of these subjects is crossed out, that means that you've contributed to it in the last 24 hours.
In your case, you posted an answer in World History about 23 hours ago
Unless you answer or edit another answer In less than one hour, the World History box that's displayed on your dashboard will no longer be crossed out.
So, to sum this up, if you contribute to a beta subject, that subject will be displayed on your dashboard under Beta Subjects.
If you come back and contribute on a daily basis, that subject will be crossed out. If more than 24 hours pass since your last contribution, that subject will no longer be crossed out.
It will still be listed as a beta subject on your dashboard, since you already contributed to it, but it won't be crossed out.