«الواجب بالأمر نوعان»
What is commanded is classified into performance (ada) and make-up (qada).
«أداء: وهو تسليم عين الواجب بالأمر»
Ada is rendering the very performance required by the command.
«وقضاء: وهو تسليم مثل الواجب بالأمر»
Qada is rendering an equivalent in place of the original duty.
Performing prayer in its allotted time exemplifies ada; making up a missed obligatory prayer is a familiar example of qada, subject to the rules governing its validity and conditions.