From LOTR-TCG Wiki
From the Comprehensive Rules 4.0:
For each wound on a character with the enduring keyword, that character is strength +2
Some of the most powerful cards with enduring keyword:
9R+48 Sauron, The lord of the rings 10R67 Ulaire Cantea, Thrall of the one 8R38 King of dead, Oathbreaker.