Need help understanding the tuple relational calculus
From Wikipedia: 

A tuple is of the form $T=(D,R,h)$ where $D$ is the domain, $R$ is a set of relation names, and $h$ is a mapping that associates a column name to each relation in $R$. To represent a tuple, we can see $D$ as the set of keys, and $R$ the table associated with those set of keys, and $h$ the column names of the table.