In the abstract, growth means "more." Its good to ask "more what?" but the "what" depends on your business/market. You should be able to ask your founders what their growth metrics are and what they look like.

If your startup is growing, it means that you as an employee will have more work to do, problems to solve, bugs to fix, features to ship, employees to help onboard, meetings to schedule, etc.

When that starts to stagnate (no new hires, no new sales, no new bug reports or feature requests, nothing new to discuss with colleagues, etc) that implies a lack of growth.