Forgive how this question may sound, but what exactly is a Scaler and what is it used for? Is it for upscaling?

=> https://en.wikipedia.org/wiki/Video_scaler

It primarily changes a video resolution to another one (up/down scaling) or adapts to a specific/custom screen resolution.
It can also convert analog video to digital (Composite -> 1K HDMI) or digital video to analog (HDMI -> S-Video), or serve as an aggregator/switch to accommodate various entries into a single output.

Some will also use algorithms/filters to improve signals/perceived quality (such as Darbee).

In the context of LaserDisc, the goal is mainly to upscale/filter/improve the Composite/S-Video signals to be displayed on modern screens and projectors that might lack analog inputs, as well as correct for aspect ratio, adjust color/contrast/etc. Ideally take both video and audio signal (analog or coax digital audio or AC3RF after demodulation) from a LaserDisc player and turn it into 1K/4K HDMI audio+video to be passed to an A/V system.

Julien

I see, I understand now. Thanks. Taking this into account, are there any recommended make/model? Lumagen Radiance 21x4?

You can find a few documented here: https://www.lddb.com/list.php?device=video&max=250

I jumped on the Lumagen Radiance 2144 boat when it was still available but people seem to have good results with newer/more affordable devices as well.

It really depends on your display, your LD player, how many other sources you have (analog/1K/4K) and how much budget you have.

Julien

used a Lumagen for many years with great results but honestly nowadays... if you own a receiver chances are good it does a good job converting the image quality into digital HDMI output. The tv upscales anyway upto it's native resolution and leaving the input resolution low gives you the proper aspect ratio control on most tv's!
just saying