A matra is one beat: the smallest counting unit of a taal. When a musician says Teen Taal has sixteen matras, they mean sixteen beats per cycle.
Matras are not fixed in clock-time — their duration depends on the laya (tempo). In a slow vilambit cycle a single matra can last several seconds and hold many notes; in drut (fast) tempo they fly past.
Groups of matras form a vibhag (section), and the full set of matras makes one avartan (cycle).