From Destinypedia, the Destiny wiki
Override Frequencies are a rare consumable added in Destiny 2, Warmind (expansion). They can be obtained by combining 4 Resonant Stems. They are used to unlock specific Sleeper Nodes hidden around Mars. When the player makes an Override Frequency the sleeper node it will unlock is random. They player can tell which Sleeper Node their Override Frequency can unlock by looking at the description which gives the player a hint about where the node is.