Endy:Victor3 plate reader/labels
A label is Perkin Elmer's terminology for a particular measurement that is taken on a well or group of wells, for example an absorbance measurement or a fluorescent emission measurement. More specifically, it is a specification of details such as the excitation and emission wavelengths used for a fluorescent measurement, the lamp energy used and the counting time etc.
For practical purposes, you create a label for a particular measurement (e.g. absorance at 600nm or GFP at 488/510nm) and then you add that label into any protocols where you want to take that measurement.
For a list of filters for different applications, see this page.