What is a NIP?
Nostr has no central server and no company that owns it, so there has to be some other way for independently built apps to agree on how things work. That way is the NIP, the Nostr Implementation Possibility. Each NIP is a short document describing one piece of the protocol: how a profile links a verifiable name, how a zap receipt is formed, how a client discovers which relays a user prefers. They are numbered, and the number is how people refer to them, so “NIP-57” points at exactly one spec without any ambiguity.
You will not install a NIP. It is a document, not a program. What you install is a client or a relay that implements some set of NIPs, and the set it supports is what decides which features actually work for you.
Why do NIPs matter to a user?
Because they are the reason two apps built by strangers can talk at all. When a note you sign in one client shows up correctly in another, it is because both read the same NIP and built to it. When a zap sent from one wallet lands and is counted by a different client, a NIP made that possible.
The flip side is honest: support is per feature, not all or nothing. A relay might carry your notes but not understand a newer NIP, and a client might implement the features its authors cared about and skip the rest. So when a feature works in one app and not another, the usual reason is plain. One of them implements that NIP, and the other has not got to it yet.