Sleeper nodes are the diamond-shaped <> nodes that are hanging around the Mars map. In Destiny 2 you can collect 44 of them. You don't collect them in order but based on what the [Override Frequency] item says. It takes 4 [Resonate Stems] to turn into 1 [Override Frequency] so you're going to need to do some farming to collect enough of them.
The hardest part is that the drop of the [Override Frequency] can be any of the 44 sleeper nodes, even the ones you have already collected. While playing, I've encountered the same node at least twice.