subset of attributes

Hi,

while reading about databases, I didn't understand what can be subset of
attributes, see the following :

Let Ri (Xi) be a relation sheme, where Xi is a set of attributes. and if t
is a tuple for Ri (Xi) and Y is a subset of Xi, then t[Y] denote the
subtuple of t corresponding to Y.

does a subset of attributes just means the instances that exist in each
tuple?

Best,