For regular users, the Discord file upload limit is 8mb. With Discord Nitro, you can increase that upload limit to 100mb with the $9.99 Nitro plan, or to 50mb with the Nitro Classic plan for $4.99.
Discord Nitro offers servers an opportunity to increase that upload speed. A Nitro subscriber can "boost" one server at a time, and more users boosting a server unlocks higher upload limits for users (including non-Nitro users) in that server. 10 users boosting unlocks 50mb uploads, and 50 users boosting unlocks 100mb uploads.