This means that if you have been rostered a day off (either legal day off or annual leave) and the company finds itself in the position of having no pilots to fly a duty they may resort to ringing you on your day off. If you elect to fly on your day off then you will get the 'day off payment' and normally get another rostered day off to replace the one you just gave up. Can be a nice little extra money earner. The payments can be anything from £100 - £400 depending on which airline and your rank.
Cheers.
A.H.