There are two types of meters used in Malayala poetry, the classical Sanskrit based and Tamil based ones.
Sanskrit meters
Sanskrit meters are primarily based on trisyllabic feet. The short sound is called a laghu, a long sound is called a guru. A guru is twice as long as a laghu. A guru is usually represented by a '-–' and a laghu by a '⌣'.
For easy specification of meters, the laghu's and guru's are grouped into feet (called ganam) as follows:
Additionally, individual gurus are also identified by 'ga' or 'gam' and laghus by 'la' or 'lam'.
For example, the definition for the meter 'Kalyani' is 'three thaganam + two gurus' indicating "- - ⌣ - - ⌣ - - ⌣ - -".
References
Malayalam poetry Wikipedia(Text) CC BY-SA