A cognate is a word that sounds like another word. A true cognate will be when the two words are the same thing, or really similar. An example would be the word 'computer' and the spanish word 'computadora'. They sound the same, and mean the same thing. An example of a false cognate is the word 'carpeta', you might think it means 'carpet', but it's actually 'folder'.
'Piano' in Spanish is 'piano'. The two words look alike, and both mean piano, so piano is a true cognate.