These are third-party apps (common on Android) that draw a digital line over the game. These range from simple static lines to complex "calculator" tools.
Most competitive players view digital overlays as cheating. However, physical rulers are a "grey area." Since a piece of plastic held against a screen cannot be detected by software, it is impossible to regulate, though it is generally frowned upon in high-level tournament play. How to Improve Without a Ruler Mini Ruler 8 Ball Pool
The most "old-school" method involves literally holding a small, transparent plastic ruler against a phone or tablet screen. By aligning the ruler with the game's short guideline, players can see the trajectory across the entire table. These are third-party apps (common on Android) that
When the target pocket is on the opposite side of the table, the default guideline is rarely long enough to guarantee accuracy. However, physical rulers are a "grey area