An account Tracker is designed to measure the engagement with any public Twitter account. Each Tracker will monitor one account, and it can be yours or someone else's, as long as it's a public account.
To begin analyzing an account, go to your Trackers listing (the bar graph icon on the top left). Click the "Start tracking" button, which will give you a drop-down menu, where you can select Twitter Account.
From there, you just need to type in the Twitter handle you wish you analyze and selecting a time zone. After several minutes, the Tracker will populate with some historical tweets from the past 30 days (typically between 1,500 and 20,000 tweets, depending on your plan level and topic). All Trackers will begin collecting data on an ongoing basis in real time once they're set up.