"The athlete who is swimming under the bridge is young."
Translation:Az a sportoló fiatal, amelyik a híd alatt úszik.
"Az a sportoló" is "That athlete", whereas "A sportoló" is "The athlete". The English is "The Athlete", but in this case there is a second clause, "amelyik a híd alatt úszik", which refers back to the athlete, so the Hungarian requires that you flag the object to which the subordinate clause refers to by making it "That athlete" ("Az a sportoló").
Note that in English, the subordinate clause immediately follows the noun that it pertains to, whereas, in Hungarian, it is generally left at the end of the sentence, so they need a method to identify which noun the clause relates to.