The flag of Sri Lanka, also called the Lion Flag or Sinha Flag, consists of a gold lion holding a sword in its right fore-paw in a maroon background with four gold leaves, one in each corner. This is bordered by gold, and to its left are two vertical stripes of equal size in green and orange, with the orange stripe closest to the lion. The lion and the maroon background represent the Sinhalese, while the saffron border and four leaves represent Buddhism and the four Buddhist concepts of Mettā, Karuṇā, Muditā and Upekshā respectively. The stripes represent the two main minorities: the orange representing the Sri Lankan Tamils and the green representing Sri Lankan Muslims.

The symbol of a lion in Sri Lankan heraldry dates back to 486 BC, when Vijaya, the first King of Sri Lanka, arrived on the island from India and brought with him a standard depicting a lion. The symbol appears to have influenced subsequent monarchs, being used extensively by them and becoming a symbol of freedom and hope: King Dutugemunu embarked on his campaign against Ellalan- an invading South Indian ruler- in 162 BC bearing a banner depicting a lion carrying a sword in its right forepaw , a symbol of the Sun and one of the Moon.

The flag was adopted in 1948 following the recommendations of a committee appointed by the 1st Prime Minister of Ceylon, D. S. Senanayake.

More Info: en.wikipedia.org