Cipher runes, or cryptic runes, are the cryptographical replacement of the letters of the runic alphabet.
The knowledge of cipher runes was best preserved in Iceland, and during the 17th and the 18th centuries, Icelandic scholars produced several treatises on the subject. The most notable of these is the manuscript Runologia by Jón Ólafsson (1705–79), which he wrote in Copenhagen (1732–52). It thoroughly treats numerous cipher runes and runic ciphers, and it is nowadays preserved in the Arnamagnæan Collection in Copenhagen.
Jón Ólafsson's treatise presents the Younger Futhark in the Viking Age order which means that the m-rune precedes the l-rune. This small detail was of paramount importance for the interpretation of Viking Age cipher runes because in the 13th century the two runes had changed places through the influence of the Latin alphabet where l precedes m. Since the medieval runic calendar used the post-13th-century order, the early runologists of the 17th and the 18th centuries believed that the l-m order was the original one, and the order of the runes is of vital importance for the interpretation of cipher runes.
More information - including diagrams - on the Wikipedia page [1]; [2] covers bind runes (which are joined by ligatures).