A gain is an increase in something, especially something good. You gain knowledge by attending philosophy lectures and reading books.
When you gain something, you acquire it or add to what you already have. You can gain friends by joining clubs, and you can gain weight by eating ice cream every day. Gain is also a noun, which you can use to talk about what you've gained, like the health gain you get from cutting back on that ice cream. The root of gain is the Old French gaaignier, "to earn, trade, or capture."