"Part" is any measure you want to use. Think interest rate. 4% APR means "4 parts per hundred per year." The "parts" could be dollars, Euros, Lira, Pesos...whatever. 4% is 4%, no matter the currency. So it is with the expansion of space: use miles, kilometers or furlongs, it is all the same. For every trillion units of distance there are this year, next year there will be 1,000,000,000,073. [Note: the "parts" cannot be light-years, because the observable universe is less than a trillion light-years across. Think of the expansion rate as a proportionality constant. If a galaxy is 50 million light-years away today, this time next year it will be: 50 Mlyr*1.000000000073 = 50,000,000.0037 lyr away. That is what 73 ppt/yr means.]