Complex sentence: Sustainable national income, (SNI) is an indicator for environmental sustainability, which gives an estimate of the production level at which-with the technology in the year of calculation-environmental functions remain available "for ever".
Split into simpler sentences: Sustainable national income, (SNI) is an indicator for environmental sustainability. The sustainable national income (SNI) in a given year is an estimate of the production level at which-with the technology in the year of calculation-environmental functions remain available "for ever".