Variables is a term used to describe data items within a dataset. So, for example, a questionnaire might collect information about a participant job (job title, whether it involves any supervision, the type of organisation worked for and so on).
This information would then be coded using a code-frame and the results made available in the dataset in the form a variable about occupation.
In data analysis, variables can be ‘dependent’ and ‘independent’, with the dependent variable being the particular outcome of interest (for example high attainment at school) and the independent variable being the variables that might have a bearing on this outcome (for example parental education, gender and so on).