The direct call feature of a two-way radio enables users to make a private call to a particular person or a group of people. While a conversation can take place between two or more people in a group, if you are on an analog radio it could still be heard by people outside your group plus it still ties up the channel everyone is on.
This feature uses the CTCSS or DCS codes of the radios to provide "simulated" privacy. These codes are transmitted sub-audibly along with the conversation and if a radio is set to receive a different code, it's speaker will remain quiet. So only people whose radios are programmed to receive the code will hear the conversation. The exception is a radio that is programmed with no CTCSS or DCS codes. It will hear all conversations.
Some radios let you choose from a list of either names or CTCSS or DCS codes from a menu selection and that allows you select from people or groups you want to talk to.
Digital radios can have actual direct call that is private and does not tie up a channel.