A piano that has not been tuned for several years will require a pitch raise.  Over time
strings lose their tension and drop in position, this causes the piano to sound out of
tune. After the piano has been tuned for the first time in a while, the strings tend to fall
back to that same position, causing the piano to go out of tune again.  By repeating the
tuning a couple of times it helps the strings stay tight and in a raised position.  The
repeated  tunings are done over the course of several weeks in order to allow the strings
time to stretch and settle.  This process is known as raising the pitch.

If after a pitch raise the piano is tuned regularly, at least once a year, further pitch raises
will not be necessary.