Variant class
About
Variant is the most important datatype of Godot, it's the most important class in the engine. A Variant takes up only 20 bytes and can store almost any engine datatype inside of it. Variants are rarely used to hold information for long periods of time, instead they are used mainly for communication, editing, serialization and generally moving data around.
A Variant can:
Store almost any datatype
Perform operations between many variants (GDScript uses Variant as its atomic/native datatype).
Be hashed, so it can be compared quickly to other variants
Be used to convert safely between datatypes
Be used to abstract calling methods and their arguments (Godot exports all its functions through variants)
Be used to defer calls or move data between threads.
Be serialized as binary and stored to disk, or transferred via network.
Be serialized to text and use it for printing values and editable settings.
Work as an exported property, so the editor can edit it universally.
Be used for dictionaries, arrays, parsers, etc.
Basically, thanks to the Variant class, writing Godot itself was a much, much easier task, as it allows for highly dynamic constructs not common of C++ with little effort. Become a friend of Variant today.
References:
Containers: Dictionary and Array
Both are implemented using variants. A Dictionary can match any datatype used as key to any other datatype. An Array just holds an array of Variants. Of course, a Variant can also hold a Dictionary and an Array inside, making it even more flexible.
Modifications to a container will modify all references to it. A Mutex should be created to lock it if multi threaded access is desired.
Copy-on-write (COW) mode support for containers was dropped with Godot 3.0.