SN stands for sodium nitrite (NaNO2). it is often used as preservation salt so it's not uncommon to come across products that have it. it's not harmful in low amounts, but it can be deadly. from what i've heard, it's really just as simple as just mixing 100g or more of it with water and drinking it, and you're going to die a peaceful death just like that. 15 minutes until you pass out, and you never wake up again.
getting it can be hard though, as people have picked up on the fact that others are using it as a suicide method, so it's a lot harder to come across, and even if you happen to come across it, then you are probably going to get police sent to your house to do a wellness check on you