Is it (theretically) possible to reduce inertial mass without reducing matter quantity? Ok, please bear with me because I only have a very little (or no?) understanding of physics outside Newtonian laws... So I was playing a video game called "Mass Effect" in which a particular compound ("Element Zero") can create a field in which objects lose what I understand is inertial mass without being shrunk down or losing matter (with this I mean that they are still physically formed by the same particles), thus rendering possible to accelerate them with a smaller force. Is it theoretically possible for such a thing to happen? Like interfering with Higg's Field?
 A: I don't think a big effect like the one in the game is possible, but a small effect is possible...
The mass of a proton plus an electron is greater than the mass of a hydrogen atom (by a little tiny bit) because $E=mc^2$, where $m$ is the inertial mass. The energy of the electron and proton in a hydrogen atom is lower than the energy of the hydrogen atom and the electron which are separated from each other. So the reduction in mass $\Delta m$ is given by  $\Delta E=\Delta m c^2$ and $\Delta m =\Delta E / c^2$. The value of $\Delta E$ is $13.6 ~eV$, which gives a value for $\Delta m$ of $\sim 2.4 \times 10^{-35} ~kg $, which is about $1.5 \times  10^{-8}~m_H$ where $m_H$ is the mass of the hydrogen atom.
So the mass of a hydrogen atom is slightly lower than the combined mass of its components. This is the only example I can think of relevant to your question.
edit after good comment from - Francesco Bertolaccini
The release of energy in fusion and fission processes can also be calculated with $\Delta E=\Delta m c^2$ - the energies released are much larger than the energy mentioned above in the formation of the hydrogen atom from an electron and a proton. For more information the wikipedia page on fusion as a power source may be a good place to start.
