An eye specialist has examined my brother's eyes as 6/18 & 6/24. what does it mean?

Needs correction: Simply stated it means your brother's right eye sees at 6 meters what the normal eye can see at 18 meters, i, e. He has to get 6 meters from something that the normal eye could see at 18 meters. The left eye sees at 6 meters what the normal eye could see at 24 meters. He needs glasses, most likely. In the 20/20 notation this corresponds to about 20/60 in the right eye and 20/80 in the left.