Base currency, display currency and automatic exchange rate imports – and why you should lock in your base currency before going live.
Magento strictly separates the base currency (the one all prices are stored in internally and all metrics are calculated in) from one or more display currencies per store view.
Basic configuration
Under Stores > Configuration > General > Currency Setup you set three values: Base Currency (the internal accounting currency, see the warning below), Default Display Currency (the default display currency for this store view), and Allowed Currencies (a multi-select of every currency customers may switch between via a storefront dropdown).
Why the base currency is a one-way street
Once orders start coming in, changing the base currency is practically off the table: every stored price, discount, tax amount and historical order total exists in that currency, and changing it afterwards would corrupt every metric you have. Lock in your base currency before going live (see also the "Initial Setup After Installation" chapter).
Updating exchange rates
Under Stores > Currency > Currency Rates you maintain the conversion rates between your base currency and every allowed display currency. Two options are available: entering rates manually right in the table, or an automatic import via a configurable rate service (the Import Service dropdown at the top of the page, e.g. Webservicex). The Import button fetches current rates and fills the table; via a cron job (System > Cron Schedule, see the "Import and Export" chapter) this import can also run automatically on a schedule instead of being triggered manually.
Customizing currency symbols
Under Stores > Currency > Currency Symbols you can override the default symbol of an enabled currency (e.g. "CHF" instead of the default symbol) – separately per store view, e.g. if your German store uses "€" and your Swiss store uses "CHF".
Practical tip: even with only one active currency, it's worth checking Currency Rates right after installation – Magento creates a rate of 1.0000 for the base currency against itself, and an accidentally different value there would silently miscalculate every price, even though only one currency is ever visible on the storefront.