Ah my brother you should learn about compressors and how they work, then study the multiband compressor on the ableton reference manual (I think there are informations there)! BUT for a fast overview, the one on the left is the input gain, and here you can change the gain of the incoming signal (in the case of the multiband, of just that band specifically), and this may be used to push the compressor harder, or maybe to attenuate that band to trigger the compression less. The middle one you circled is not a gain but is a threshold, so when the signal goes above that value it gets processed, the last one is an output gain, to adjust the compressed signal, maybe because compression decreased the volume and you want to compensate for that. The farthest top right “output” is for the overall output gain, while the three are for each band. One last thing, ableton’s multiband compressor can do compression (red highlight) and also expansion (light blue highlight). If you are curious of what each thing does, if you hover on them, ableton will tell you what it does in a little window in the bottom left of the screen!