Adjectives modify or describe a noun (person, place, or thing), usually by ascribing it with some specific attribute that limits or restrains its meaning. These descriptive words might provide details about the noun's size, shape, type, color, feel, or quality.

In the case of "a walk through the woods on a summer day," you would want to first identify the nouns before deciding how to modify them. In this case, "walk," "woods," "summer," and "day" are all nouns. But there is also another noun inferred here: the person doing the walking. How does the person feel during their walk? Are they having a leisurely stroll, or are they lost and trying to find their way out?

Any additional nouns having to do with any of these things might also be ideal for modification by an adjective—for instance, "moss" growing in the "woods." How do these descriptors impact the scene? Your adjectives could paint a picture of a walk through hostile woods that is slowly turning dire just as easily as they could be used to describe a peaceful, carefree walk through an idyllic grove.

Each adjective should be chosen to limit the scope of how the reader sees the object. The rocks among the trees are not just rocks, but jagged rocks. They're not just woods, but dense woods. Adjectives are used to paint a more detailed picture for the reader.