It has to do with the Hot Network Questions List (see the sidebar on the right side of the site with questions from many different questions from across the SE network).
The "Hotness" of a question is basically,
(sum of Q + A score) / time since asked
Also in order to be considered, a question needs at least one answer.
Once a question hits the list it pulls in more users from across the network who will add more answers and upvotes perpetuating the cycle.
As far as what gets a question onto the list it seems like,
Questions can get huge numbers of views without the HNQ by being useful to a lots of people.
As far as how you can tell the hotness of a particular question there is a user script that will try and estimate it for you. I have found it to be mostly accurate and it looks like this,
As you can see on the right there is a percentage estimator with 0% being not hot and over 33% being hot enough to get onto the list.
As for why this would be useful to know,
If you have hit the rep cap from a Q or A on the list, it can give one an idea of how long it will be tell it drops off the list.
If you are going for points, hotter questions get more votes. Its kind of sad how one person can post 5 good answers that don't make the list and end up with less rep than a user who only posted one question.
If you are trying to clean stuff up to be presentable, the Hot Questions are where a lot of users are going to land.
Tips for making the list,
- Good descriptive titles.
- Post it during times when lots of users are around which means during the week and between 6 UTC and 23 UTC and preferably closer to 6-10 UTC. (6 UTC is when the users in India come online and 23 UTC is quitting time in California)
- Slightly wider topics so more people answer.