Strainers are extremely dangerous!
A strainer is an obstruction in a river that only limited amounts water can pass through.
The best way to think about a strainer is to visualise paddling down a section of river that is being pushed through a colander that has loads of holes in it but none of them are big enough for a person or boat to pass through. A strainer can be anything from a tree that is stuck into some rocks to a car that someone may have dumped in the river.
Strainers are always bad so make sure you stay well away from them.