At a glance, the query timeline tells you where an agent is in their query queue, as well as other information like how much they're requesting or rejecting and the length of time between the submission and the response. Although not everyone marks their queries in QueryTracker (obviously), it still provides a good snapshot of what you can expect.
Here's an example of an agent who has been extremely timely about replying to queries in the past 30 days. The red frowny faces are rejections, but the short length of the line indicates that at least they're quick rejections:
And here's someone who I wouldn't expect to hear from for a while:
This timeline, on the other hand, shows me at a glance that this agent is a quick responder and is actively requesting material (the smiley green faces). She's moving up in my queue!
While some might feel that this level of detail is a little too obsessive, I find that it actually helps me stop obsessing. Instead of wondering why I haven't heard back from someone who seemed to be such a good fit for my manuscript, I can see at a glance that the agent is a month behind on queries and stop worrying about it.
The query timeline is a feature only available to Premium members, but like I've said before, QT Premium membership is inexpensive—only $25 a year—and totally worth it for querying writers. You can find out about the rest of the Premium features here. No affiliation, just a happy customer!