Tuning involves tightening and loosening the tension of the steel strings in a piano, so that they are organized to sound beautiful, or in harmony, together. The higher the tension of a string the higher the pitch of the sound, and conversely, the looser the string the lower the note of that string will be.
Each string is attached to a steel tuning pin which is embedded into wood (pinblock). The piano tuner uses a tuning lever on these steel tuning pins to raise and lower the tension.
In most pianos there are more than 230 strings and all must be properly tuned to each other to sound musical.