A Floating Point Number is a way to represent a precise number on a computer system. The "floating point" refers to the decimal point. Other than this it is very similar to scientific notation.  Read more about floating point numbers.

An expression is made up of operators and operands.  They preform different comparisons or calculations used to evaluate a single value.  We often use different expressions to for various reasons in PHP.  Read more.

e is a mathematical constant, and an irrational number.  It's value is approximately equal to 2.7, and is used in several PHP functions included exp (x) which calculates e to the power of x.  Read more about e.

Comparison operators are used to compare one value to another.  There are four different ways you can do this:  greater than, less than, equal to, or not equal to.  Each is represented with a different symbol.  Read more about comparison operators.