SDES stands for SDP Security Descriptions for Media Streams.
When keys needs to be exchanged for the use of SRTP sessions this can be done in multiple ways. SDES is one of them.
The keys to be exchanged are placed in clear-text inside the SDP. It is assumed that the SDP is then being sent on top of another secured transport such as TLS.
It is also assumed that end-to-end security is not necessary, as signaling here may traverse through an intermediary server who has access to the SDP information.
SDES has been explicitly banned from use in WebRTC. DTLS-SRTP is to be used instead.