const means that the data cannot be changed.

Code:
const unsigned char in[] = "qwerty";
in contains the characters "qwerty" which cannot be changed.

Code:
unsigned char in[] = "qwerty";
in again contains the characters "qwerty" but the characters in the array can be changed.

Code:
const unsigned char in[] = "qwerty";

unsigned char in2[] = "qwerty";

	in[0] = 'z';
	in2[0] = 'z';
In the above, the line highlighted produces a compile error as the code is trying to change the q to a z but in is defined as const so changes are not allowed. The in2 change is allowed as in2 is not defined as const.

Hopes that helps.