It is better to be content with what one has than to try to get more and risk losing what one already has. (It's better to have a small actual advantage than the chance of a greater one).
The earliest English version of the proverb is from the Bible and was translated into English in Wycliffe's version in 1382, although Latin texts have it from the 13th century:
Ecclesiastes IX - A living dog is better than a dead lion.
http://www.phrases.org.uk/meanings/64950.html
Cf. 13th-cent. L. plus valet in manibus avis unica quam dupla silvis, one bird in the hands is worth more than two in the woods. Parodied by the American actress Mae West (1892-1980) in the 1934 movie Belle of the Nineties: �A man in the house is worth two in the street.�
It is more sekyr [certain] a byrd in your fest, Than to haue three in the sky a-boue.
[c 1450 J. Capgrave Life of St. Katharine (EETS) ii. iii.]
Betyr ys a byrd in the hond than tweye in the wode.
[c 1470 Harley MS 3362 f.4]
http://www.answers.com/A%20bird%20in%20the%20h and%20is%20worth%20two%20in%20the%20bush