What does ^ stand for in this expression?

This is an exponent, or "to the power of".
Seven to the power of seven to the power of seven to the power of seven. Man, that is a BIG number!

to the power of

7^7 = 7*7

x^x^2 = (x)^(x^2)

7^7^7^7=7^(7^(7^7))

7^7^7^7= [ {(7^7)^7}^7]

5^2=25
3^4=81

It means "to the power of".

e.g.
2^3 = 2³
3^2 = 3²

To the power of.

You get quite a big number with that.

squared