English, an array is a collection.
In C also, it is a collection of similar type of data which can be either of int, float, double, char (String), etc. All the data types must be same. For example, we can't have an array in which some of the data are integer and some are float.
Suppose we need to store marks of 50 students in a class and calculate the average marks. So, declaring 50 separate variables will do the job but no programmer would like to do so. And there comes array in action.
datatype array_name [ array_size ] ;
For example, take an array of integers 'n'.
int n[6];
n[ ] is used to denote an array 'n'. It means that 'n' is an array.
So, int n[6] means that 'n' is an array of 6 integers. Here, 6 is the size of the array i.e. there are 6 elements in the array 'n'.
We need to give the size of the array because the complier needs to allocate space in the memory which is not possible without knowing the size. Compiler determines the size required for an array with the help of the number of elements of an array and the size of the data type present in the array.
Here 'int n[6]' will allocate space to 6 integers.
We can also declare an array by another method.
int n[ ] = {2, 3, 15, 8, 48, 13};
In this case, we are declaring and assigning values to the array at the same time. Here, there is no need to specify the array size because compiler gets it from { 2,3,15,8,48,13 }.
Every element of an array has its index. We access any element of an array using its index.