The amino acid citrulline was first isolated from watermelon, thus its name was derived from the Latin word citrullus, meaning watermelon. In the body, it is produced during the conversion of L-arginine to nitric oxide (a vasodilator which causes blood vessels to get bigger to increase blood flow). Malate or malic acid is found in many sour fruits (e.g., green apples, sour grapes) and it gives them their tartness. In the body malate is important for ATP (energy) production. Citrulline malate (CM) is a compound formed by the chemical boding of citrulline to malate. Supplement scientists have been interested in this compound because of its potential to increase ATP (energy) levels and blood flow in working skeletal muscle---resulting in less fatigue, harder workouts, and faster recovery.
Recent research published in the Journal of Strength and Conditioning Research provides evidence that CM supplementation improves muscle endurance and recovery. When CM was ingested 1 hour before bench press exercise, subjects completed nearly 53% more reps at 80% of their 1 rep max and reported 40% less muscle soreness 24 and 48 hours after the chest workout. This is remarkable considering this was after a single dose of CM!

Pérez-Guisado J, Jakeman PM. Citrulline malate enhances athletic anaerobic performance and relieves muscle soreness. J Strength Cond Res. 2010 May;24(5):1215-22.