max(x, y)

Calculates the maximum of two numbers.

x: the first number, any data type

y: the second number, any data type

The larger of the two parameter values.

sensVal = max(senVal, 20); // assigns sensVal to the larger of sensVal or 20
                           // (effectively ensuring that it is at least 20)

Perhaps counter-intuitively, max() is often used to constrain the lower end of a variable's range, while min() is used to constrain the upper end of the range.

Because of the way the max() function is implemented, avoid using other functions inside the brackets, it may lead to incorrect results

max(a--, 0);   // avoid this - yields incorrect results
 
a--;           // use this instead -
max(a, 0);     // keep other math outside the function

* min(x, y) * Constrain

Source: arduino.cc