We have two currencies, TZS and KES (Currency A and B). 1 shilling of currency A equals 20 shillings of currency B, it means that currency A is stronger than currency B.
In this context, currency A has more purchasing power compared to currency B. This means that with 1 shilling of currency A, you can buy more goods or services than you can with 20 shillings of currency B. So, in terms of purchasing power, currency A is stronger and more valuable.
Petrol in Kenya is around Ksh 200. With 200 shillings of TZS cannot buy the same goods that KES can buy. So Kenya shilling has a higher Purchasing power than TZS since you can buy more goods with 200 shillings of KES than 200 shillings of TZS.
If Purchasing Power ingekuwa comparison would have been how many goods in comparison to the dollar can a currency buy, then TZS would have been stronger than the EURO and Pound, since with 10 Dollars (in Euro) you buy way less goods than with the same 10 dollars in TZS.
Simple economics hamfunzwi shuleni?