ResearchNet is a part of the game where one can create and play custom assignments. It is unlocked after completing The Blue Danube assignment on the Atropos Station, although it can be accessed at anytime without unlocking.
ResearchNet also happens to be the name of the subreddit /r/researchnet that is used by people to share their ResearchNet assignments. The assignments are shared as a string of characters. By copying those characters to the clipboard and entering the Import menu in the ResearchNet of the game, the assignment is imported. /r/researchnet should not be confused with /r/spacechem/ which is a general SpaceChem subreddit for everything but sharing levels.