It is the e mathematical constant, Euler's constant.

why is eulers constant important

Assuming it is not a variable, e is also known as Euler's Constant.

Wait a minute, Euler's Constant is something different. e is known as Euler's number (which is about 2.718...) or sometimes known as the base of the natural logarithm. Most maths people and examiners should know what 'e' is though, so you can use it in formulae safely. It's called Euler's number to prevent people getting confused with Euler's Constant. Euler's Constant however is 0.5772... (it's actually called the Euler-Mascheroni constant) which is signified by a backward 'r'. I mean no disrespect towards to previous answerer. Hope I helped