To inhale is to breathe in. It is the opposite of "exhale," which is to breathe out.
When we inhale, we draw air into our lungs through our noses and mouths. Then we exhale, or breathe the air out again. If we do this slowly, as in yoga or meditation, it can be quite relaxing. However, some things like smoke, toxic fumes, or germs might make us sick if we inhale them. And if we accidentally inhale water while we're swimming, it makes us sputter and cough. This is because our lungs are only interested in good, clean air.