The  calcium ions bind with troponin subunits. Hence, the masking of the active sites of myosin is removed.

What s meant by dis????? Where s d active site??????/

The active sites which are meant for binding of the myosin head is present on the actin filament. The troponin subunit is present over the actin filament and covers the active site. Calcium ion binds to the troponin and displace it thus exposing the active site present on actin. Now myosin can interact with this active site. Myosin pulls actin filament towards each other. This results in muscle contraction.