I have this code, but I fail to understand what "other" actually is, and what it's trying to do.
public interface Comparable<T>
{
int compareTo(T other);
}
What does the parameter "other" suppose to mean?

It's just a parameter name, not a keyword. It's the other value you're comparing "this" value to. So suppose you're comparing two people, you might have:
Person fred = new Person(...);
Person george = new Person(...);
int result = fred.compareTo(george);
It would be up to the compareTo method to compare fred and george (which it would know as this and other) by whatever means it deemed appropriate, e.g. age, name etc.
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