Operators are words or symbols that, as suggested by the name, operate on values (their operands). Most operate on two, but some operate on only one (unary). Familiar ones are the four basic mathematical operators: '+', '-', '*', and '/'.
Operators are not just limited to number operands, others work on strings (concatenation operator) or other datatypes. C++ has a sizeof operator that returns the size in bytes of some data type, and Lua has the length operator '#' that returns the length of a table or string.
Operators can also work on bitwise values, and may be spelled out as 'and' and 'or', or use symbols like '&&' and '||' for the same purpose.