If two sides of a quadrilateral are congruent and parallel, then the quadrilateral is a parallelogram.
A parallelogram is a quadrilateral with opposite sides parallel and congruent. If two sides of the quadrilateral are parallel, then the other two sides are also parallel (since opposite sides of a quadrilateral are parallel), and if two sides are congruent, then the opposite sides are also congruent (since opposite sides of a parallelogram are congruent).
Therefore, a quadrilateral with two sides congruent and parallel must be a parallelogram.