かるく おして みて。
Try pressing it lightly.
Literally: “lightly + try pressing (=press [and] + look [and]).”
Irresponsible teacher that I am, I didn't explain why the i-adjective 軽い (かるい) is changing to 軽く (かるく), an adverb.
As in this example, we turn i-adjectives into i-adverbs by changing the final い to く.
In the case of na-adjectives, we just put に after them.
For example, 静か (しずか // quiet) is a na-adjective.
Put a に after it, and you get 静かに (しずかに // quietly):
しずか に あるいて。
Literally: "quiet / silent + に + walk (and)."