The thinking level decides how much effort ChatGPT puts in. The levels run from Instant through Medium, High, and Extra High to Pro, depending on your plan. Higher levels think longer, handle harder problems, and get a bigger context window, but they take more time and use more of your allowance.
Instant is right for most everyday work. Raise the level when a problem has teeth: a tricky analysis, a long document, a decision with many moving parts. On Business there is also a Higher intelligence setting.