"A TL-whaa?" you ask.
TLR is short for Twin Lens Reflex. These cool-looking cameras were marketed for decades, in various designs, and reached the pinnacle of their popularity in the 1960s. You can often find vintage TLRs on the used market at affordable prices - eBay, Craigslist or the odd yard sale. They typically run around $50-200, depending on the model and condition. If you're lucky, you may find one for a few bucks since the general public assumes film is no longer "a thing." Popular makes include Rollei, Yashica and Mamiya.
So how does a TLR work? Unlike an SLR, where you view and shoot through a single lens, the TLR uses one lens for viewing and another for taking the actual shot. This separation of lens functions has a variety of practical implications, one of which is that very close objects may require adjusting the camera position due to a phenomenon known as parallax. For shooting at normal distances (6+ feet, or so), this is not a problem. Due to the simplicity of their design and robust construction, these cameras also tend to hold up well over the years. Most use a single, fixed focal-length lens although there were some standard accessory filters and close-up lenses made.
TLRs typically use a waist-level viewfinder, meaning you look down from above to compose and focus. Some models include a magnifying glass that flips out for more precise focusing (I use mine regularly). Because the scene is projected directly up from a mirror in the body, the image is flipped left-to-right, which can be a little disorienting at first. The viewing screen is surprisingly bright. In fact, there's a whole group on Flickr dedicated to using digital cameras to shoot through these viewfinders. Of course there's no reason you can't still use these cameras as intended to take photos on film - which, as you might suspect, is exactly how I use mine.
Most TLRs use 120 film, which is readily available online in a variety of types. Some models took 620 film, which is the exact same film on a thinner spool. 620 film is no longer made, but you can order re-spooled film for a small premium cost, or buy used or newly-manufactured spools online and do it yourself. You can buy color, slide or black & white film for that authentic old look. Loading film in your TLR is even easier than popping the back off your smartphone to install a micro-SD card. You can usually find videos on YouTube demonstrating the use of a particular vintage camera so there's plenty of community support readily available out there if you find yourself unsure of what to do.
So why would you want to shoot with one of these vintage cameras? For starters, they shoot square photos that yield the beautiful, authentic look of film. The lenses are generally quite sharp, so your photos look at least as good as anything you shoot for Instagram. The negatives are several times larger than 35mm film, which means you can enlarge your photos to a high degree - really, we're talking massive enlargement potential. Another feature I enjoy is the "swirly" bokeh (out of focus area) effect I sometimes get shooting at larger apertures on my Yashica.
They're also great conversation starters, as many people have never seen a camera like this in the wild, and older people appreciate the nostalgia. It's unusual for me to not have at least one person ask about my TLR when I use it in public!
If you're not already a film shooter, but are looking for a cheap way to dip your toes into the water, keep your eyes open for a good deal on used TLRs the next time you stop at a yard sale or flea market. You might score a great buy on a beautiful and "obsolete" camera. Just don't tell the vendor that you can buy film for it.