Variables are a way of temporarily storing data. This is important to understand because variables will not persist when you reopen your blend file. Take a look at properties for this.
Variables can also not be displayed in the interface.
In Serpens, variables are attached to a specific node tree. While you can access variables across node trees, try to embrace the temporary nature of variables and use them for specific operations instead of all over your node trees.
You can create variables in the Serpens N-Panel.
Variable Types
Data
Can store any type of data. You will need to define the data type when plugging this into a different type of socket as Serpens needs to know the actual data type to properly convert it. You will rarely use this data type.
String
Stores text.
Boolean
Stores either True or False. You can imagine this as a checkbox. To actually display a checkbox use a property.
Float
Stores a decimal number
Integer
Stores a number.
List
A list of data. The data in this list can be of any type, even more lists. You can add data to a list and remove it. To access data use a for loop or the Index List node.
Pointer
Collection
Nodes
The get, set and reset variable nodes can be used to change any variable.
For list variables there is also the ability to add items and to remove or sort them. Take a look at the list nodes for details.