SSI is a very common abbreviation. It stands for Support Service Inmate. Sometimes you will also see SSIs described as janitors, but SSIs can work in other support details as well, such as johnny sack (sack meal) distribution.
A janitorial SSI position can be a sought-after job because it allows the SSI access to other prisoners, sometimes across multiple housing areas. This can be an opportunity to pass along information and kites, and for trade, all of which can benefit the SSI both in status and in property.