A clean boosts is a pedal that will boost the volume of your guitar signal, with practically no tone coloration, or adding much if any distortion from the pedal.
It is also important to understand that the pedal itself is not the most important thing that dictates how clean sounding your clean boost will be; it is the pickups in your guitar, and the amp that you use. An amp with a large headroom will stay clean and if it has a character of breaking up early then it will be pushed into overdrive sooner at lower volumes.
Clean boosters are also excellent to boost the overdrive/distortion pedals for more crunch and sustain.
A clean boost has a bunch of benefits that are hard to get from your average overdrive pedal. IF a clean boost is pinned with a guitar that has medium to high output pickups, they can achieve a creamy sounding clean overdrive that is very hard to achieve with a standard overdrive pedal.
There are several designs of boost pedals like, BJT based, OpAmp based, JFET based, boost with buffers etc.
Its usually great to discuss & consider the need of the guitarist & design accordingly. Insigniahifi offers a range of boost effect units & all are made to & customized to orders.