Simplify the following text for a Grade 4 native English-speaking person.
Original: Hybrid Theory was a massive commercial success ; it sold more than 4.8 million copies during its debut year , earning it the status of best-selling album of 2001 , while singles such as `` Crawling '' and `` One Step Closer '' established themselves as staples among alternative rock radio play lists during the year .
Simplified: It sold 4.8 million copies in the first year , most of these were in 2001 . The singles of the album are `` One Step Closer '' , `` Crawling '' , `` Papercut '' and `` In the End '' which made the band famous .