From Eterna Wiki
The sequence, or primary structure, of an RNA molecule is the list of ribonucleotides that it contains. That is, it is the order of the RNA's nucleotides. A sequence is customarily written in the 5' to 3' direction unless indicated otherwise.
For example: AAGUGCUCAGAU
Indicates that the order of nucleotides on the RNA from 1 through 12 is two Adenines, a Guanine, a Uracil, a Guanine, a Cytosine, a Uracil, and so on. Eterna allows you to manually type in a sequence when solving puzzles.