Calculating the power of a signal
The formula is a variation on: Root Mean Square (RMS). It's missing the root function.

From Wikipedia:   



If you summed values without squaring them, a symmetrical waveform would have zero power. Squaring the samples avoids this.

The 1/N sigma stuff is the arithmetic mean. This is your equation.

By taking the square root, you remove the nonlinear distortion caused by squaring your original samples. You may wish to double-check with your tutor: accidents can happen when transcribing equations (no finger pointing!)