The Codex Sinaiticus is a Greek hand-written manuscript of the Old and New Testaments. It was written in the 4th century, in uncial letters. It came to the attention of scholars in the 19th century at the Greek Monastery of Mount Sinai, with further material discovered in the 20th century, and most of it is today in the British Library.
For many the whole area of Bible Texts and original manuscripts is just an unintelligible minefield. To help you understand more about the CS I recommend the following links: