Opa!
Basically the difference is in the point of view in relation to the class to which this analysis will be made.
Explain:
Let’s take as an example the class "Example"
Efferent coupling: Let’s say the example class needs to send an email and post some information in an API. If you have a bad design, this class will have efferent dependency to these two classes:
class Exemplo{
ClienteEmail clienteEmail;
ClienteApi clienteApi;
}
That is, the Example class has efferent coupling to the Clienteemail and Clienteapi classes.
Afferent coupling are the classes that use the "Example class", example:
class Aplicacao{
Exemplo exemplo;
}
class Teste{
Exemplo exemplo;
}
That is, both Application and Test have matching to the Example class.
I know they were Toy examples and I know how not applicable these examples are, but I hope to have elucidated a little your doubt.
A more detailed explanation you can find here:
http://leandrodaniel.com/index.php/code-metrics-parte-3-medindo-acoplamento/