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][1] commented, this is called "[Binnen-I][2]".

In opposition to [Jens Krügers][3] 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.][2]:

> Anfänger_innen

> Anfänger*innen

> Anfänger(innen)

> Anfänger/-innen <sub>[repeated to get a visual "complete" list]</sub>

> Anfängerinnen und Anfänger (as "ladies first")

> Anfänger/ Anfängerinnen

> Anfänger (as "[generic masculinum][4]")

"The normal way" is an ongoing debate in german society.


  [1]: https://german.stackexchange.com/users/5770/sebastian-redl
  [2]: https://en.wikipedia.org/wiki/Binnen-I
  [3]: https://german.stackexchange.com/a/52781/36160
  [4]: https://de.wikipedia.org/wiki/Generisches_Maskulinum