TDS = Total Dissolved Solids.
TDS is basically 'anything' that is in the pool water. This can include salt, calcium, buffer, acid, even the nasty things kids can do in swimming pools.
TDS can often be confused with the 'Salt' level in the pool and often pool shops may consider these to be one of the same. Most chlorinators rely on large amounts of salt and minerals to produce chlorine for the pool. The Naked System works on TDS which is why salt levels can be up to 90% less than traditional pools giving you that rejuvenating and refreshing experience during and after swimming.