A temporary nanny is someone that you use as a nanny at short notice, or for a short period of time. It could be that, for example, you are having to go into hospital for an operation and therefore you will need a nanny to look after your children for a short period whilst you are recovering. Or it could be that you need to cover for your regular nanny having a problem of their own and not being available. The advantage of a temporary nanny is that they are normally available at much shorter notice and though they will have been screened and vetted properly by the agency that you use, there will be less time between you needing them, and them helping you out.
The temporary nanny could be either full time, or part time and is fully flexible depending on your needs. It could be that you just need them to deal with specific times and dates, or maybe you just need them whilst you go on holiday, to look after the children, or for a special occasion. There is a lot of variety. The temporary nanny will normally be able to provide most of the functions of regular full-time nanny, such as driving the children, preparing meals, washing and ironing clothes, and doing household shopping as well as assisting with housework.
Because of the temporary nature of temporary nannies then you should expect to pay a higher salary on the basis of an hourly rate for them. This is because they do not have security of tenure in their positions and so have to make up for it in salary. You could expect a temporary nannies salary to be as much as double, if not higher, than that of a full time nanny doing the same job.