Strontium (Sr) is a chemical element with atomic number 38. It is part of a group of elements called alkaline earth metals.
A major use of strontium in the recent past was in CRT TVs (thick TVs) where it prevented X-ray emission. Nowadays these displays are being replaced by flat screens which work differently and don't require strontium. Another use of strontrium is in fireworks, where it can be used to give the explosion a red color.
Like calcium, which is also an alkaline earth metal, strontium is part of what makes up our bones. Also, there are four stable strontium isotope whose relative proportions vary by geographic location. An interesting result of these facts is that it's possible to determine the origin of a person or animal based on their bones, something which is useful when studiying ancient migration patterns.