"E o idee buna"
Exchange tax is a percentage charged when citizens exchange one currency for another (for example converting gold to the local currency). It is taken from the amount being converted and goes to the country budget.
Raising it earns the country budget more from every exchange, but makes converting money more expensive for citizens, which can discourage trading currencies.
Lowering it makes currency conversions cheaper for citizens and can encourage more exchanging, while the country budget earns less from each one.
This is a neutral explanation to help you decide — it doesn't favor either side.