Showing research experience in graduate applications
You're expected to describe research experience. It sounds like you've had some. You should describe it in your application. Talk about what role you had, what sort of things you actually did ("various stuff important for the project" and "working with Matlab" aren't very good descriptions). What was your role? Did you make decisions or did you just assist? What sort of products did you produce and what skills did those products require? How did you solve problems you encountered?

The idea behind these application questions is to try to differentiate people with no experience from some minimal experience (cleaning instruments in a lab and feeding rats) to moderate experience (sounds like what you have) and people who engaged in truly independent research (which is pretty rare, depending on where you did your bachelor degree and in what field). Be honest and detailed, that's all that's expected. Of course people will play up what they did, but if you made substantive contributions that demonstrate your abilities and experience, that will show through in your write-up.