What is a vowel-zero alternation in Linguistics/Phonology?

I'm working on an ignment for my Phonological ysis cl and I have a data set that I have to create rules for to explain why they change from the underlying singular form to what they are in the plural form. We are told right away that this data set shows a vowel-zero alternation and I'm supposed to create a rule for that, but I have no idea what a vowel-zero alternation is and cannot find a definition for it anywhere. Any help is greatly appreciated! Thanks!