The noun strain is a stretch, effort, or exertion. You can strain your ankle, or your little brother can strain your patience.
This word has many, many meanings. Athletes strain their knees, which strains the emotions of their fans. Usually, you have to strain a little to accomplish anything, like getting an A in math. A strain may also be a line, like a line of ancestry, of thought, or a melodic line in music. Maybe you have to strain to hear the quiet flute in an orchestra. Another meaning is to pass something through a filter, like tea leaves.