Learning to read complex math formulas
The following points may be helpful:
$i$ is used to index the various numerical values $x_i$ you have. Usually, unless specified otherwise, it is understood that $i$ ranges from $1$ to some finite value $n$.
Thus, in your example, you have $n$ observations each one of them is denoted by $x_i$.
$\bar{x}$ denotes the mean of the $n$ observations i.e.,
$$\bar{x} = \frac{x_1+x_2+\ldots+x_n}{n}$$
The same interpretation holds for $y_i$ and $\bar{y}$.
$\Sigma$ stands for sum and hence we could have re-written the mean as follows:
$$\bar{x} = \frac{x_1+x_2+\ldots+x_n}{n}=\frac{\Sigma_i{x_i}}{n}$$
I hope that helps decipher what is going on in the equation.
For this particular formula, any introductory statistics book will be a good start. Conceivably you won't have the background for this book, and then you would need to backtrack.
In particular, for this question you could look up "sigma summation notation," as that is the only really strange symbol here. The other symbols, such as $\bar x $, would need to be defined. The formula you asked about happens to be the formula to calculate Pearson's correlation coefficient, measuring how much change in one variable corresponds to change in the other. In this case, the values come from data points on the plane, and having a bar means an average (so $\bar x$ means the average of all the $x$).
This comprehensive pdf could be useful: Handbook for Spoken Mathematics
It represents the mathematical symbols and explains how to read them.
for example:
https://ibb.co/cjturF