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Given the following post, respond as if you're commenting on Reddit:
human
ELI5:What is supergravity / supersymmetry?
gpt
Quantum mechanics divides particles into two types that obey different rules, fermions and bosons. Supersymmetry claims that, for every particle species, there is a partner species such that, of the two, one is a boson and the other's a fermion. If this is true, electrons (which are fermions) are partnered with selectrons (which would be bosons), and photons (which are bosons) are partnered with photinos (which would be fermions). In physics, "symmetries" are things that don't change the laws of physics, the idea being that, if you swap all particles with their partner species, the interactions are unchanged. (Well, not quite unchanged; the details depend on particle masses, and these undiscovered partners must have a lot more mass or we'd have found them by now.) But supersymmetry is the only unconfirmed symmetry nature can have, given the constraints of quantum mechanics and relativity.

Particles and their supersymmetric partners have similar interactions with the Higgs field. There's a technical problem with the theory of Higgs bosons that can be easily solved if high-mass supersymmetric partners exist. And some attempts to explain gravity in the same terms as other forces in nature require supersymmetry to work. Supergravity is one such approach; another is string theory.