Currencies appreciate when capital wants in: higher relative rates, stronger growth, an improving current account, or a flight into a safe-haven-currency during stress. None of it requires official action, which is what separates it from revaluation.
Appreciation is not uniformly good news domestically. It cheapens imports and helps contain inflation, but it prices exporters out of foreign markets and reduces the local value of foreign earnings, which is why finance ministries complain about strength as often as about weakness.
Measured against one partner it says little; measured against a basket it says more, which is the purpose of a trade-weighted-index.
Example: USD/JPY falls from 155.00 to 140.00. The yen appreciated by about 9.7%, since 1/140 is that much more dollars per yen than 1/155. A Japanese exporter's $1m of revenue becomes ¥140m instead of ¥155m.
Related: depreciation, revaluation, trade-weighted-index, safe-haven-currency