It is pre-defined formula.

Excel provides 15 groups of functions.
Formula Tab–>Function Library group

syntax of function
=funcName(arg1,arg2,….)

arguments are values on which function has to work.
arguments can be numbers, cell address or cell range.
Many arguments (max.255) Eg: SUM, AVERAGE

3 ways to enter a function:
Direct typing into cell
(or)
Select desired function from Formulas tab–>functions library

Some of the important Excel Functions:
SUM
adds the values of given arguments

syntax:
=SUM(number1,[number2],…)

AutoSum: Alt+=
AVERAGE
returns the mean value of given arguments

syntax:
=AVERAGE(number1,[number2],…)

MIN
returns the minimum value in the given arguments

syntax:
=MIN(number1,[number2],…)

MAX
returns the maximum value in the given arguments

COUNT
returns the number of numeric values in the arguments