The sliCount property of the RTCInboundRtpStreamStats dictionary indicates how many Slice Loss Indication (SLI) packets the RTCRtpReceiver for which this object provides statistics sent to the remote RTCRtpSender.
An SLI packet is used by a decoder to let the encoder know that it's detected corruption of one or more consecutive macroblocks (in scan order) in the received media.
In general, what's usually of interest is that the higher this number is, the more the stream data is becoming corrupted between the sender and the receiver, requiring resends or dropping frames.