When you enroll in a plan on the Health Insurance Marketplace, you are agreeing to share costs with an insurance company for the services you use. Each month, you pay the insurance company a premium to retain your coverage. Once you start using health services, you are typically spending toward the deductible (the minimum you must pay out of pocket before insurance kicks in).
Once you begin to share costs, you are paying co-insurance (a portion of the overall bill) towards things like diagnostic or imaging services. Once you hit your out-of-pocket maximum for in-network services, your insurance company typically covers the full cost of approved care.