Psi is a form of energy. There are many types of energy, core, soul, and chi, to name a few. Core is similar to psi but is more affective but harder to use, soul is powerful but not recommended for to use because it can cause problems when messed with and chi is life energy witch which is also important to your functioning. There is a theory that all energies are the same one, but for the sake of sanity this article will assume there are different types. Different types of energy have different attributes. Some types explode on contact, and others suck other energies in. The energies that are used for general manipulation have two main attributes, effectiveness (how well it works) and workability (how easy it is to use). Psi is very easy to work with and fairly effective.
Psi will feel different to different people some feel it as warmth; while to others it feels cold. Some feel a magnetic repulsion, while others feel an attraction. All in all only you can really know what it feels like. The best way to feel psi is to get someone to send you a chunk to feel.
There are many ideas and theories on why humans can manipulate psi, but there is no fixed theory. The best thing to do is come up with you own.