This is the 811th most frequent Czech word.
bratr
Brother.
In this sentence, 'bratr' is used in its nominative form as the subject of the verb 'je' (is). It straightforwardly means 'brother'.
Můj bratr je o dva roky mladší než já.
My brother is two years younger than me.
Here, 'bratrovi' is the dative form of 'bratr', as it indicates the recipient of the help ('help my brother').
Chci pomoci svému bratrovi s úkolem.
I want to help my brother with his homework.
In this instance, 'bratra' is in the accusative form of 'bratr', which is used when the noun is the direct object of the verb 'viděl' (saw).