Japanese script uses Chinese characters.Usually, they have two (or more) ways of being read; there's a Japanese one (kunyomi) and a Sino-Japanese one (onyomi), which reflects the way people of the time thought characters were pronounced originally. When you put two different characters together it's likely that they will be read with the onyomi.

And so, you have 平 with the kunyomi "Taira" and 平家, "House of the Taira"; onyomi, "Heike". In the same way 源 "Minamoto" and 源氏 "Lineage of the Minamoto", Genji.