If you've done sufficient Arduino programming, you'd have seen that there are two ways of defining constants.
One way is to use #define, like
#define const_name 3
The other way is to use the const keyword, like
const int var_name = 3;
#define is like a placeholder. The Arduino compiler replaces all mentions of this constant with its value at the compile time. This means that the values defined using #define don't take up any program space.
Variables defined using const, on the other hand, are just normal variables, whose values can't be changed. They take up program memory space, and have a type (which is advantageous in many situations).
In general, it is preferred to use const over #define, for defining constants.