Part of learning magic is to understand the power of names.
When a character casts certain spells (such as ones that summon, bind, create, scry, command, ward or banish) they can chose to use either a True Name or Secret Name in the casting.

A True Name is one or more word that is intrinsically bound to your very being. It is used by spells like Sending to identify it’s target.
A true name may be the one you were born with, or the one that shaped you into who you are today.

True Name’s can be stolen, sold or borrowed and must be spoken or written without error for to work – Fey, Fiends and Devil’s are all known to highly prize such things and they are used in the making of Pacts and deals between two or more entities.

In some cultures the name a PC one is born with remains secret while a ‘translation’ (usually in common or undercommon) is used outside of family members and those most trusted companions.

For others they may have more then one true name and use these and various titles to make the process of ‘naming’ more complex… as to fully bind them requires using each name and title in the correct order to work – it must be done perfectly or else you risk a incomplete binding or mis-naming where you summon or bind yourself to some other entity instead.

If you know a True Name you can use it to target a willing creature or make a arcana or performance check to use this when casting a spell that can target this type of creature.

The DC is set by the CR rating/Level of your target and each incorrect or missing name increases the DC by 5.

If successful: You have advantage on your spells/saves and ability checks to summon, bind, create, scry, command, ward or banish and they have disadvantage on attempts to escape the effects and refuse your commands

As a action you can attempt to Command the creature using it’s true name using a performance or arcana check.
The DC is set by the CR rating/Level of your target and each incorrect or missing name increases the DC by 5.

A Secret Name is the word spoken at your point of creation, it is in the language Primeval.
Most beings in the universe do not know their secret name, and to learn one grants total control over the the creature.

Fun Fact! Spells that grant ‘total control’ over their target like Find Familiar and Find Steed draw upon the power of Secret Names!