An IIP is a proposal to update some function or standard within the Index Coop. IIPs can vary widely from proposing new Index products, to requesting funds from the Index Treasury, to performing Index Coop smart contract upgrades. The IIP proposal process largely follows the same behavior as Ethereum's EIP process.
The Index Coop GitBook (what you are reading) is the central repository and source of truth for the status of IIPs.
This is a high level overview of an IIP lifecycle:
Formal Proposal on the Index Coop Forum
Discussion & Minimum 48 Hour Waiting Period
IIP Proposer Schedules Snapshot Vote
Implementation by the parties involved with the improvement