A node is the visual representation of a function. To make life easier, Unreal Engine 4 provides the ability to create logic without writing code, by just using and connecting nodes.
A node can have inputs, outputs, both, or, in some cases, nothing except the execution ports.
For example, a PrintString node is a visual representation of the typical printing function, but has even more parameters. You can pass a string value to the In String input, or a Boolean value to the print options, or even a color variable to TextColor; however, as you can see, the nodes have no outputs. The white in and out arrows are not considered inputs or outputs, they are the function start and end, which represent the curly braces in a normal function.
Another example can be a node like Event Hit, which is an event node having no inputs, and usually has no execution as it executes itself. However, as you can see, it has lots of outputs, which always vary based on the current situation of the hit event.