I see that term thrown around in a lot of threads. Obviously this means non-desirable non-converting traffic, but what are the real reasons for low quality sources?
Visitors of the site are poor maybe? The site traffic is full of bots? People don't trust the site? The nature of the site itself isn't good for ads? Visitors are banner blind because of a zillion other ads on the site already? Other stuff? Any of these?
I'm just pulling this out of my mental cosmos but I'd split this into two psychological categories:
Intent, and Expectations.
Intent: how much motivation or desire the users have for the product or service based on where they came from and what where they doing. Where they watching porn? Or where they socially engaging during a boredom period? Or where they searching for a specific solution to a problem? Or browsing a website in some specific genre?
Where they come from has a lot to do with their mindset and what it is that they desire, and this has a profound impact on their value to an advertiser.
Expectations: what the user believes a product or service is going to offer them. A lot of this will come from your angle, ad copy, banners/landers i.e. the funnel itself. How do you frame the offer? How do you make it appeal to the user? How do you sell. Whatever you do, you will set up some expectations in the user's mind. Do you use the word free? Do you claim the product does something which is a stretch of the imagination?
Whatever the case, the path traveled to the offer that you have influenced will effect how the user behaves on the back end.
Just my $0.02.
"Low quality traffic" depends on the context. Obviously bot traffic for every advertiser. But dating advertisers see under 25 year old traffic as low quality. But the 20-24 yr old bracket could be a gold mine for another advertiser.
Quality = purchase intent
Let's take an example:
High Quality Traffic: Let's say you are selling a web hosting package. Then high quality traffic would consist of people who are actively in the market to purchase web hosting. This is generally why Google search traffic is considered high quality. When someone goes on Google and types in "web hosting in Dallas," chances are pretty high that this person has intent to purchase web hosting in Dallas.
Medium Quality Traffic: This may be people who were not actively looking to buy web hosting, but perhaps through targeting or good convincing ad copy, they have chosen to at least find out more about them. The conversion rates on these individuals will be lower than those who have already made up their mind to buy web hosting, bit chances are that a decent percentage of these people will convert.
Low Quality Traffic: This may be people who clicked on an ad because they thought that they were going to win an iPad, or because the ad showed scantily clad images of the opposite sex, or because they fat fingered on an ad even though they were not intended to. There will be very little intent to buy web hosting from this crowd. In order to get this to work, you will need to play the numbers game and buy so much ad space that the very small number of people who may buy the product pays for the costs of all the other ads you buy.
Fraudulent Traffic: Many people confuse this with low quality traffic, but fraudulent is different from low quality traffic. Low quality traffic at least will have some (albeit extremely small) level of purchase intent among the people who click on your ads. Fraudulent traffic will not. Fraudulent traffic thus is not low purchase intent traffic ... it is no purchase intent traffic. These are bots or fake traffic or other types of traffic whose sole purpose is to screw the advertiser and get money from you.
Basically traffic that doesn't back out for the advertiser.
For dating it's under 25 traffic generally, for mobile carrier billing offers wifi traffic and so on.
As Mr Green said, what is bad quality for one offer can be great quality for another.