Xel'naga relics were pieces of xel'naga technology left on the surface of Zerus. They served as observation devices as the zerg were manipulated by Amon and his followers into their purity of essence. They housed genetic information within them.
When Sarah Kerrigan arrived on Zerus and sought to destroy the primal zerg pack leaders, she discovered these devices scattered around the planet. Abathur noted that the genetic information within them would be helpful in improving the swarm.