It is a gendered plural of "Anfänger und Anfängerinnen". So both (or all, would be off-topic here) gender male and female are addressed.
As Sebastian Redl commented, this is called "Binnen-I".
In opposition to Jens Krügers answer I'd like to point out, that
Anfänger/-innen
is not "the normal way".
Because in my observation there is no general way to address all gender in one or more words. Others are e.g.:
Anfänger_innen
Anfänger*innen
Anfänger(innen)
Anfänger/-innen [repeated to get a visual "complete" list]
Anfängerinnen und Anfänger (as "ladies first")
Anfänger/ Anfängerinnen
Anfänger (as "generic masculinum")
"The normal way" is an ongoing debate in german society.