Usages of der Tisch
der Kantinentisch
the canteen table
ein Tisch
a table
Er setzt sich an einen kleinen Tisch.
He sits down at a little table.
an (at) is a preposition that can work with both dative and accusative case.
Accusative is the case of movement towards a direction.
an einen kleinen Tisch
is accusative
He is moving into a sitting position at the table.
Don't worry at this stage too much about the forms. We will tackle that later, but try to grasp the idea of when to use the accusative.