Complementarity is achieved through interactions between nucleobases. In the RNA, base complements are adenine - uracil (A-U) and guanine -cytosine (G-C). RNA secondary structure is achieved when two regions of one strand base pair and form a double helix.
Respecting the rule of complementarity and base-pairing, some of the bases will pair and some will not, as it can be seen on the image. The stability depends on helix length, base composition, and the number of mismatches.