The symbol for approximately equal is a squiggly equals sign. It is used to show that two numbers are roughly equal, but not exactly equal. Typically, the notation is used in an expression like this.
In plain language this means that the constant (tau) is approximately equal to . In reality, there are many more significant digits for this constant. For example, the author could have written this instead.
As an alternative to using the approximately symbol, sometimes author’s will use dots to mean the same thing.