Can only shop during a specific enrollment period.
With most other insurance, it only pays out of something unplanned happens. With health insurance, there are medical issues that can be known about in advance of them being to be addressed. So you might know you have a heart condition that needs to be operated on soon, but not immediately. This is known as a medical precondition.
Before the ACA was passed, health insurance companies would always exclude medical preconditions. So, if you switched insurance while needing that heart operation, you would find that you weren’t covered and have to pay all the costs.
The ACA got rid of being able to limit coverage of medical preconditions, but it needed to provide a way for insurance companies to limit their exposure to people switching from a bad plan to an amazing plan that covered everything and would have to pay out immediately. To handle that, it made it a requirement for all people to get a minimum amount of medical insurance and to restrict when people could shop for insurance.