Someone that does more than just the work assigned to them by someone else. It's largely more about how much influence you can be expected to have than a hard delineation in technical competence.

These are actually pretty well defined at the lower to mid levels. This is a ball-park.

- IC3: Executes well on what is assigned to them by their manager. Requires direction. Does only what is told. Frequently an up-or-out role. "New Grad"

- IC4: Identifies proactively what needs to be done and works with their manager to justify and schedule it. Beginning to show signs of independence, but requires managerial guidance. Frequently an up-or-out role. "Junior Engineer"

- IC5: Identifies proactively what the team needs, and leads a cross-functional effort involving 2-3 individual contributors to complete it or solves the hard problems. Effectively autonomous, low blast radius. Terminal role. "Engineer."

- IC6: Identifies proactively what the nearby organization needs beyond just the specific team, and leads a cross-functional effort involving 4-6 individual contributors, PM, marketing, design, etc, to solve those needs -- and solves the hardest problems. Effectively autonomous, high blast radius. Terminal, well-paid role. "Staff Engineer."

The poster sounds like an IC3, and is likely to be paid like an IC3 until they can demonstrate their chops.

At a start-up especially one without an engineering ladder, you start as an IC5. You will need to perform like one, as there's not a whole lot of room for folks who don't.

FWIW, these numberings vary from company to company but expectations are roughly in line industry wide. Even if your company doesn't have a ladder, you should act like it does because when you company gets one, you wanna be up top.