What do you mean by implicit none in Fortran?

In Fortran, there is a feature of Implicit typing which means there is no need of declaring the variables before the use of the variables so the first letter of the variable name will determine the type of the variable. i, j, k, l, m, n are taken as integer numbers and other as real numbers.

The excellent programming practice is that the variables should be declared at the starting of the program so implicit none statement disables the implicit typing so that variables can be announced at the starting of the program