Translation:I play the piano.
The function of は is to mark the topic, which is not necessarily the subject – it can be just as well be the object (or various other roles). So 「ピアノはひきます。」 is a legitimate sentence.
As I understand it, one might say that 「ピアノをひきます」answers the question "What do you play?" (or "What do you do?") and 「ピアノはひきます」 answers the question "What do you do with the piano?".
You could say ha/wa , if you want to indicate that you play ´pianos´ in a general statement. Piano ha/wa hikimasu.
It is not wrong, just a difference structure that emphasizes that you are talking about pianos as the subject, instead of indicating that ´piano´ is one of the instruments that you are able to play.