This is the 11481st most frequent Polish word.
jednemu
"Jednemu" means "to one" in English.
In this sentence, 'jednemu' is the dative form of 'jeden', meaning 'one', used to refer to a specific individual indirectly.
Pomogliśmy jednemu chłopakowi naprawić rower.
We helped one guy fix his bike.
Here 'jednemu' again is dative, indicating the one person receiving the object in a specific context.
Najpierw musisz dać jednemu klucz, dopiero potem możesz kontynuować.
First, you need to give one of them the key, only then can you proceed.
In this case, 'jednemu' is used in the dative case, referring to one person out of a set (here 'us').