An important part of learning the piano (or learning about many types of music in general) is understanding and recognising chords. The most common types of chords in the Western world are major and minor triads: three-note chords that are built by stacking notes that are a 3rd apart. Chord progressions provide harmony in music, adding colour to melodies by blending different notes together in various combinations. At the bottom of this post is a downloadable PDF I have made of major and minor chord flash cards so that you can print them as a memorisation aid.
When it comes to learning and remembering certain concepts in music, a lot of input and repetition is required to allow the information to sink in and begin to make sense. Key signatures are a good example of this – with twelve major and twelve minor keys, it can seem like a daunting task to memorise the correct sharps and flats needed for each scale. A lot of my students like using flash cards for learning this kind of information and testing themselves on their retention.