Supply teaching is when you are asked to cover a classroom as a teacher. Supply teaching can be done for a short period i.e a day or even half a day or it may be required for a longer terms, such as to cover maternity or long term sick leave for example.
A supply teacher will often find that their role varies from school to school and often even from class to class meaning that sometimes they will be working off lesson plans that have already been given to them and other times they will be responsible for planning the lessons, teaching them and also marking the work afterwards.
Supple teacher are often paid more per day than permanent teachers but it may be that they one get work for a few days of the week so that money needs to last them longer. Also as a supply teacher you will not be paid for holidays unless you are on some sort of a contract which states you will.