Synergies are a feature in Diablo II introduced via patches. The general idea is that skills not only grow in power as players invest more and more skill points in them, but also enhance other skills of the same Skill Tree, mostly by increasing damage or some other stats.
Some synergies work two ways, that is, each point in Fire Ball will also increase the damage of Fire Bolt by 16%. The amounts given to each skill may or may not be same. Sometimes, upgrading a skill via Synergy may prove more effective than putting skill points in it.
In a manner of speaking, this system allows the characters to increase the power of their skills far beyond the normal Level 20 limit (actual maximum of hard points in one skill). It also allows more diversified builds.
The Skills needed for the respective synergies can be seen at the very bottom of every skill's information in the Skill Tree.