A protocol parameter change is a governance action that adjusts one or more of Cardano’s on-chain parameters: transaction fees, block and script size limits, economic constants, or the governance thresholds and deposits themselves. Parameters take effect without new node software, which distinguishes this action from a hard fork initiation.
It is voted on by DReps and the Constitutional Committee; for parameters classified as security-relevant, SPOs vote as well. Proposed values must also pass the guardrails script (see new constitution or guardrails script), which bounds what a single action is allowed to change.