The word strain has several meanings:

1. To injure a muscle or body part by stretching it too far or using it too hard. If you strain your back lifting something heavy, you've pulled or damaged the muscles. Athletes sometimes strain their hamstrings by running too fast. A strained muscle hurts and needs time to heal, but it's usually less serious than a broken bone or torn ligament.
2. To make a great effort, often a physical one. You might strain to lift a heavy box, strain to see something far away, or strain to hear a quiet voice across a noisy room. When your little sister strains to reach the top shelf, she's stretching and pushing herself as hard as she can.
3. To separate liquids from solids by pouring through a filter or mesh. When you strain pasta, you pour it through a colander to let the water drain away. If you're making fresh orange juice, you might strain out the pulp and seeds.
4. A type or variety within a species. Scientists study different strains of bacteria or viruses, like how flu shots target specific strains of influenza each year.

The word can also mean tension or pressure: The strain of studying for finals tired everyone out. Something under strain is being pulled, stretched, or stressed.