Long variables are extended size variables for number storage, and store 32 bits (4 bytes), from -2,147,483,648 to 2,147,483,647.
If doing math with integers at least one of the values must be of type long, either an integer constant followed by an L or a variable of type long, forcing it to be a long. See the Integer Constants page for details.
long var = val;
var: variable name.
val: the value assigned to the variable.
long speedOfLight_km_s = 300000L; // see the Integer Constants page for explanation of the 'L'