Ratio

A user's ratio is a number that is calculated based on the how much the user has uploaded and download. To find your ratio, you divide your upload amount by your download amount.

As a formula: $$Upload / Download$$

Most trackers will show your ratio in your user bar or on your profile page.

Why Ratio Matters
Many private trackers enforce a ratio requirement which means you must maintain a certain ratio or else your accounts will be disabled or put on a watch list. The required ratio varies from tracker to tracker and usually is very lenient for your first few gigabytes but typically required ratios range from 0 to 1. Some trackers also choose to go ratioless or ratio-free meaning they don't enforce any sort of ratio requirement, this is generally paired with seeding requirements that ask you to seed for a certain amount of time.

Improving Ratios
Improving a ratio can vary in difficulty depending on the tracker, generally the more users the easier it is.

On some trackers the only way to increase your ratio is to transfer content to users. Good ways to do this are to upload your own content to the users, rent a seedbox to seed popular files, or jumping on popular torrents early (consider using an RSS feed or a script like Autodl-irssi.

A number of trackers employ a bonus/seed point system which rewards users for doing things like seeding, uploading, commenting, and participating in contests. These points can sometimes be used to purchase upload credit which adds to your upload amount thus increasing your ratio. This system promotes good retention and helps users with slow connections.

The two methods above are the most common however there are sometimes other miscellaneous ways to gain upload credit such as joining their IRC, joining contests, playing games, and bidding on games/events.

If none of the above options seem viable then the last way to improve your ratio is simply just seeding for long amounts of time and waiting for users to download. This generally feels slow at first depending on the popularity of your torrents but after you have a lot of seeding files you will gain a healthy stream of upload.