The principle is very simple.
On a gridlined sheet, you need to check the empty squares.
A cross mark shows the square that you selected.
The goal is to align your signs next to each other, horizontaly, verticaly or in diagonal.
As soon as 3 to 5 side by side symbols form a line, you are awarded one point. The opponent is obviously looking to do the same.
As a wise strategist, you too will aim to block your opponent from marking points by positioning your symbols in such a way.
The first to reach the required number of points is the winner.
Once you selected a square, its the opponents turn to check an empty one which will marked with a circle.
And like this, once at a turn the squares are being filled.