A dipstick is a tool which can be dipped into liquid to measure how much of it there is, or to test it for things.
Measuring dipsticks show the amount of liquid in a container. These containers are usually hard to access. Measuring dipsticks are used by inserting and removing the stick and then checking how much of the stick is covered by the liquid. The oil level dipstick is an example of a measuring dipstick found on most internal combustion engines.