Drivable short-term, but fix now to avoid damage.
This code means the engine computer found a fault in the electrical circuit for ignition coil I, which provides spark to one of the cylinders. The usual culprits are a failing coil, damaged wiring, or a loose or corroded connector. It's a common, well-understood code that is typically fixed by replacing the bad coil and addressing any wiring issues.
$100 – $350
Varies by vehicle and root cause.
Short trips are usually fine, but it isn't safe to rely on long term. A misfiring cylinder can overheat and ruin your catalytic converter, leading to a much bigger bill. If the engine runs rough or the light flashes, stop and have the car towed.
Expect roughly $100 to $350 in most cases. The ignition coil itself is affordable and quick to swap. The price rises if connectors or wiring need repair or if a technician recommends replacing several coils at once.
It's worth taking seriously. A single misfire won't immediately strand you, but left alone it can damage emissions parts and make the engine run poorly. Plan to fix it promptly rather than ignoring the warning.
The 'I' is simply a label that identifies which ignition coil is reporting the fault. Your technician uses the factory wiring diagram to map coil I to its specific cylinder so the correct coil gets replaced.