The board is a grid of 6 by 6 dots. That makes 5 by 5 boxes, so 25 boxes in all, and 60 lines to draw before the game is over.
Six dots across and six dots down. The dots frame 25 boxes in a 5 by 5 grid, and 60 lines join them: 30 across and 30 down. Every one of those lines is drawn once, by one player, before the game ends.
Why 25 boxes
An odd number of boxes means the final count can never be level, so there is no draw. Border lines touch one box and inner lines touch two, which is what lets one line close two boxes.
The lines are named by the two dots they join, so the top-left line is A1–B1. For a plan to use the space well, see the Dots and Boxes strategy guide.