Runic alphabets are pre-Roman alphabets used widely across Europe, easily recognizable because of their angular characters. There are many different varieties of runic alphabets, of which the most well known is the so-called Elder Futhark (the name is derived from the sounds of the first six characters).
Runic alphabets are still used today in many mainstream and non-racist contexts. However, white supremacists have also appropriated the runic alphabet, in large part because Nazi Germany often used runes in its symbology. White supremacists use runes for transliterated Roman letters, creating an alternative alphabet (sometimes viewed as a code, since the vast majority of people do not know runic letters).
It is also common for white supremacists to use runic letters to portray certain words of significance to the white supremacist cause, which may be seen on clothing or as tattoos. Among the most common words or phrases translated into runes are "skinhead," "white power," "hate," and "RAHOWA."
Because runes are still commonly used in a variety of non-racist forms, their appearance should always be carefully analyzed in context.