A tea strainer is a device that is placed over a cup and catches tea leaves as tea is poured through it. It employs a fine mesh to filter out the leaves.
When tea is brewed the traditional manner in a teapot, the leaves are not contained in teabags. This means they are freely suspended in the liquid. As the leaves themselves may be distasteful, it is usual to filter them out with a tea strainer.
Strainers usually fit into the top of the cup and hold the leaves as the tea is poured through them, when you brew in a pot.
This is a double-layer fine mesh strainer, it works better than regular ones.