Calcium is one of the more common elements, and is the most important metal in most living things, mainly on account of its abundance. It's used in animals as a signaling ion, especially in nerve tissue, and also is the principal cation (a cation is a ion having a positive charge and characteristically moving toward the negative electrode in electrolysis) that makes all animal hard parts, be they apatite (bone), carbonate (shell), or almost anything else. It's particularly important in geology as well as being the primary element that binds to carbonate ion thus keeping most of the Earth's carbon dioxide out of the atmosphere (otherwise we would be like Venus). The reason why I hesitate to say that calcium is the most biologically important metal is that magnesium is an essential part of chlorophyll, which is the pigment that makes photosynthesis possible.

Calcium is the 5th most common element found in the Earths crust and the 5th most common ion found in seawater! Calcium is also the most common mineral in the human body, most (99%) of which is located in your bones and teeth.