I had Rhotacism when I was younger. I went through many sessions of therapy to correct it just to find out that I physically couldn't pronounce 'r' because my tongue was too short to reach the back of my mouth to pronounce the 'r' sound. They wanted to perform oral surgery to make my tongue longer, but then I would have looked like Gene Simmons.
I'll try to explain again, it was already said as sibilant, but in layman's terms I would describe it as a 'hiss' like that of a snake. It's just an 's' that is accentuated. It's easier to just see someone differentiate the sounds, but the 'hiss' comparison is the closest I can get to an explanation without making a video of myself pronouncing words with a sibilant 's'.