Where the timing patterns are
A QR code has two timing patterns. One runs horizontally in row 6 between the top-left and top-right finder patterns, and the other runs vertically in column 6 between the top-left and bottom-left finder patterns, counting rows and columns from 0.
Each one alternates dark, light, dark, light, and starts and ends with a dark module. Because the sequence is fixed, it carries no data of its own.
What they do
The timing patterns act like a ruler. By counting the alternations, a reader learns the exact module pitch and the number of modules per side, which confirms the version it estimated from the finder patterns.
This matters most for larger codes and for images where the print has stretched or the camera sees the code at an angle. Micro QR codes have timing patterns too, along the top and left edges.