This is the 14223rd most frequent German word.
harter
"Harter" is the comparative form of "hart" in German, meaning "harder" or "tougher" in English.
Here, 'harter' is an adjective describing 'Schlag' (hit), meaning 'tough' or 'severe'.
Dieser Film war ein harter Schlag für das Boxbüro.
This movie was a tough hit for the box office.
In this instance, 'harter' modifies 'Test' (test) to signify 'challenging' or 'demanding'.
Das Training gestern war sehr hart, ein wirklich harter Test für uns.
The training yesterday was very hard, a truly tough test for us.
In this example, 'harten' is the declined form of 'harter' to fit with 'Winter' (winter, masculine noun), conveying 'harsh' or 'severe' in context.