It's simple enough to understand 'it gives mass to particles', but you've got to think about it further which brings me to these questions...
- What is the higgs field? how do they know this exists? or is it just a mental framework to explain what is happening?
The higgs mechanism theories primary effect is to give mass to the exchange particles of the weak interaction. The weak interaction is very similar to the electromagnetic interaction in some ways, but different in others. One of those differences is that the weak interaction exchange particles have mass while photons (the exchange particles of the electromagnetic force) are massless (travel at the speed of light, which is a trivial fact because photons are what you call light).
The higgs boson might be understood as evidence that this theoretical concept (of the higgs mechanism) might actually be real (and not just a mental framework to explain whats happening).
Hit wikipedia and read about string theory, and if you want to know more about it, you can reply and ill try to explain things a bit more in depth, but a better source for information about these topics would be Hawkings popular books for example.
- Why is the higgs boson the only particle that interacts with the higgs field?
That question is based on an invalid assumption (that the higgs boson is the only particle interacting with the higgs field). The higgs boson is something that sometimes gets created when something reacts with the higgs field. Its like in a nuclear bomb where uranium decays into lots of other elements, and some of which have a really short half-life, but you want to detect one of those. Youre not looking for what caused the bomb to explode, but this particle existing is evidence for a bomb having been detonated.
- If the higgs boson has a mass\energy of 125GeV, how can it add varying degrees of mass to other particles? is a heavier particle made up of multiple higgs particles?
As explained above, the higgs boson is just a byproduct of a process. By measuring the higgs boson, physicists think theyre finding evidence of that process happening. The higgs boson is not the particle that sits in every atom giving mass to them. It has a very short half-life (and is thus not stable).