A number contained in or derived from your data. Dimensions and metrics in Data Studio reports come from data sources.
Metrics are aggregations or calculations, such as a sum, (X+Y), a count (how many X?) or ratio (X/Y). If your data set contains web analytics information, you'll have metrics like Users (the count of users who visited your site, Conversions (how many converted—another count), and Conversion % (a ratio: conversions/users).