I guess that E is a reference to scientific notation. It stands for *10^ as in 2.5E6 which means 2.5*10^6 or 2.5 million or 2,500,000. The number after E can be negative: 2.5E-2 means 2.5*10^-2 or 2.5/100 or 0.025.
E could also be a variable in an expression or equation.
Lowercase e is the base of natural logarithms and can be seen in expressions and equations as ln(x) or e^x.