Loosely fill the cup with leafy herbs. Pressing them down will result in a much larger quantity than intended, potentially overpowering the flavor of your dish.
When a recipe calls for a cup of loosely packed herbs, it's referring to the volume the herbs occupy without being compressed. Leafy herbs have a lot of air space between them. If you firmly press the herbs into the measuring cup, you're essentially forcing more herb material into the same volume. This means you'll end up with significantly more herbs than the recipe intends.
The impact on flavor can be substantial. Using too many herbs can easily overwhelm other ingredients, creating an unbalanced and potentially unpleasant taste. Delicate herbs like basil can become bitter if used in excess. For best results, gently pile the herbs into the measuring cup until it's full, without applying any pressure. You can even lightly fluff them up a bit to ensure they aren't settling too much. Level off the top with a knife or your finger for an accurate measurement.
If you're unsure about the correct amount, start with slightly less than the recipe calls for. You can always add more herbs to taste, but it's difficult to remove them once they're incorporated into the dish.