Consecutive numbers means that they are one after the other (i.e. 1, 2, 3 are consecutive numbers). The square of a number signifies the product of this number times itself (i.e. 2 times 2 = 4, so 4 is the square of 2). If these squares differ by 25, then when they are subtracted from one another, the difference will be 25:
12 x 12 = 144
13 x 13 = 169
169 - 144 = 25