The sam [pronounced sum] is always the first matra of an avartan. For example, in teentaal, the first matra, Ta, is the sam.
The sam is usually stressed when playing and is particularly important when playing in groups. For instance, in Kathak, in a piece in teentaal, the tabla player provides a steady rhythm by playing theka, while another musician improvises. When it is time to switch roles, the two musicians can easily coordinate by using the sam as a marker, since it is the beginning of the next avartan.