As dimension, you can use any field that exist in your data model.
That's where you can click on a field name and 'Add' to the list of dimensions on dimension tab.
But you can also do a transformation of a field, a so called calculated dimension.
For example, your data model only contains dates, but you want to group by Year:
=Year(DATEFIELD)
could be used as calculated dimension then. There are more complex calculated dimensions, but in principle, there are all similar in providing something 1-dimensional (like an array of values) to group your record set.
