This is the 9168th most frequent Russian word.
сережа
"Сережа" is a diminutive form of the male given name "Sergey" in Russian.
Serezha is a male first name here, commonly used informally to refer to a person in Russian.
Сережа учится в пятом классе.
Serezha studies in the fifth grade.
Here, the possessive form 'у Сережи' indicates an entity belonging to Serezha, who is referred to using the genitive case.
У Сережи новый планшет.
Serezha has a new tablet.
The phrase 'С Сережей' uses the instrumental case to show a relationship between the speaker and Serezha in the context of the action.