It could mean knit the back sts, or knit back across the sts you did in the previous row.
To dec sts evenly means to space the decs evenly apart. They don't absolutely have to be exactly even - sometimes the st numbers don't work out that you can k6 k2tog all across the row. Just don't bunch up the decs all at one end, both ends, or in the middle. This page has a calculator, and the site also has one for increasing sts too.