-- The arrow in the picture is pointing to a crest of the wave ... one of the peaks.
-- The amplitude of the wave is the vertical distance between one of the crests and the light horizontal line across the middle.
-- The wavelength is the horizontal distance between two crests.
-- Do you know what a 'trough' is ? Well first of all, when you read the word, it looks like you should pronounce it like "trow", but the correct pronunciation is "trawf". (Just like when you have something stuck in your throat, and you cough ... you don't "cow", you "cawf".)
In an old cowboy movie, do you remember the big, open water tanks they used to have along the side of the street in front of the saloon, for horses to drink from while they were tied up to the hitchin post ? THAT tank was called a "watering trough". The bottom of a wave, where the wave could hold water, is the trough of the wave. It's an upside-down crest.