What is [Serialized]?

I can not understand [Serialized] keyword. Please your kind and excellent answers.

This is not a C# keyword but an attribute - a kind of custom, programmable keyword-like thing. Attribute is a meta-data class, so one can tag classes, methods or fields with it to provide additional information about it to your/other programs.

[System.Serializable]

Do you mean [Serializable] ?

System.Serializable is a attribute read by serialization programs (XML, JSON, etc) to signal that this type is to be serialized.

SerializableAttribute Class (System) | Microsoft Learn

[UnityEngine.SerializeField]

[SerializeField] is similar as it signals to the internal Unity serialization system that given private field is to be serialized (value saved as part of scene or prefab file).

Unity - Scripting API: SerializeField

Thank you…And Could you explain this [SerializeField] float speed;
I have some unclear points. I am looking forward to your kind reply.

Thank you for your clear solution.