Compensatory rest is an equivalent rest break your employer must give you at another time, when your job genuinely doesn’t allow you to take your normal uninterrupted rest break, for example if you’re the only person covering a site.
It still has to be a real, restful break of the same length as the one you missed, not just a promise on paper that never actually happens.
In practice
A security guard working alone overnight can't take an uninterrupted 20-minute break because there's no one to cover the site. Their employer has to give them an equivalent rest period at another point instead, not just let the entitlement disappear.
Legislation covered