Most of the time, to use a service online, you must create an account with the organization that provides the service. If you want to use a social media application or create a bank account, you need to provide information on yourself for the organization to grant you access. This is the centralized model: identities are stored and managed on one central organization's servers. This is totally incompatible with the concept of blockchain that promotes decentralization to achieve mass-coordination without a central entity.
So, how do we create identities that are not controlled by any third party in that kind of ecosystem?