Well, my holidays were nice: I attended a course titled analysis and object-oriented design with UML at UEL given by Prof. Sergio Tanaka and climbed the Pico Agudo (Sharp Peak) with Dylan, Gustavo and Lucas.
A sequence diagram shows how groups of objects operate between them and in what order.
You can design a sequence diagram describing the actions of the actors (such as press buttons or select options) and the consequences of these actions (such as get some data from database or set options), which become operations of classes and boundary classes.
In complex systems it is important because if you need to make changes or adaptations, it is easier to identify where to make it with the sequence diagrams of your system.
Finally, after you finish designing the sequence diagram and the relationships between classes, you can generate the Java code of these classes.