These patterns represent where a user's eyes flow as they read through a document, page, book, etc. Optimising your design for these paths helps you emphasises the content you'd like.
Before using it's worth noting that different patterns work for different types of content. Consider what you're creating before applying these.
For text-heavy content. Four quadrants, with a 'reading gravity path' from top-left to bottom-right. If something isn't in either of these two quadrants, it may receive less attention.
Follows the shape of the letter 'Z'. Start in the top-left, move to the top-right, then diagonally to the bottom-left, before finally reaching the bottom-right.
Follows the shape of the letter 'F'. Start in the top-left and move to the top-right. Then bounce back to the top-left before moving down, and shifting to the right again. Each sweep towards the right will be shorter than the one before it. Akin to a 'scanning' pattern.