system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Fundamentally, what is mass?

If you want go even further how are the particles that have no mass?
gpt
There are two types of mass, which as far as we know turn out, for reasons unknown, to be exactly the same:

 - Gravitational mass is the quantity that determines how much gravity something has. According to general relativity, gravity is caused by objects trying to take the shortest possible path through spacetime: if their route through spacetime is bent, their path is altered, and we observe that as gravity. The more massive something is, the more it bends spacetime, and so the more gravity it exerts. Objects with no mass don't bend spacetime, and so don't gravitationally attract other objects (although they can be attracted by objects with mass). (edit - correction: objects with no rest mass still bend spacetime because they have energy)

 - Inertial mass is the measure of how resistant an object is to acceleration. Heavier objects require more energy to accelerate to the same speed than a lighter object would. In a vacuum, massless objects travel at the speed of light without any force acting on them (although it's possible for objects with mass to travel faster than massless objects in certain mediums). 

There's no known fundamental reason why these two masses end up being the same thing, as they're two entirely different concepts. However, the fact that they are the same is one of the fundamental facts which general relativity relies on. 

Fundamentally, mass (both gravitational and inertial) comes from two places. First, via E=mc^2 , you have the mass resulting from the energy that binds the smaller particles together (e.g, gluons binding quarks together in a proton). For constituent particles like protons, neutrons, or atoms, this is the source of the vast majority of mass. Second, you have the mass of a particle resulting from its interaction with the Higgs field. The Higgs field is a field that permeates the entire universe and interacts with particles proportional to their mass: particles with a lot of mass interact with the Higgs field a lot, and particles with no mass don't interact with it at all. This interaction is mediated by the Higgs Boson.