It's a prefix to indicate the number is in hexadecimal rather than in some other base. The C programming language uses it to tell compiler.
Example:
0x6400
translates to 6*16^3 + 4*16^2 + 0*16^1 +0*16^0 = 25600.
When compiler reads 0x6400
, It understands the number is hexadecimal with the help of 0x term. Usually we can understand by (6400)16 or (6400)8 or whatever ..
For binary it would be:
0b00000001
Hope I have helped in some way.
Good day!