Calculating the power of a signal
As @jojek has already said in the comments, the formula reads
$$
P_\mathrm{x} = \dfrac{1}{T}\sum_{t=1}^{T}x^2(t)
$$
As $t$ usually denotes continous time I find this formulation a little bit odd. What your tutor probably actually meant is
$$
P_\mathrm{x} = \dfrac{1}{T}\int_{t=0}^{T}x^2(t) \mathrm{d}t
$$
It calculates the average power of the (continous-time) signal $x(t)$ of length $T$. In computer programs we're dealing with discrete-time (and discrete valued) signals, of course and the average power of the digital signal $x_n$ is given by
$$
\tilde P_\mathrm{x} = \dfrac{1}{N}\sum_{n=1}^{N}x_n^2,
$$
where $N$ is the length of $x_n$ in samples.