Boost? Overdrive? Pre-amp? I don't know, maybe all of that, depends in how you use it.
For me is a "always on" pedal - more than volume and/or gain, it adds texture and grain to the sound.
I use it as a "pre-amp", my amp is a single (clean) channel "pedalboard amp" (seymour duncan powerstage 170) and all my gain and eq, modulations and what not effects come from my pedal stacks. So I was missing a "tube-like" feeling on my sound. This pedal compensates that, for me at least.
I can set bass, treble, volume and gain, and these allow me to work with the amp settings and give it the extra "layer" I was missing in my sound. With these you decide how to use it - to boost a solo with volume or gain (depends if your distortion comes before or after in the pedal chain), to "crunch" a clean part or, to give a permanent texture to your sound, stacked with an effect or not.
In my case I use between distortions/overdrives, so I can get more volume, presence and grain from both my direct/clean sound and the "before" distortion pedal (when on), but also adding gain on my "after" distortion pedal (when on) - this one used for my "boosted" leads/solos.
Very affordable and with great results on my sound!