It means there is some growth of a tumor through the wall of the
prostate.  It is of little consequence if the surgeon cuts beyond the
extent of the prostatic extension thereby removing all the tumor, it is
of more consequence if the surgeon cuts through the extension leaving
some behind.  The patient then has a "positive margin" which will have
an impact on the likelyhood of recurrence depending on the amount of
cancer left behind.
Dave Perry

In general, "extraprostatic extension" means that the cancer extends
beyond the prostate gland.  It is is the opposite of organ confined.

As already noted, prostate cancer can still be cured even if the cancer
is not organ confined provided the part that extends beyond the gland is
either destroyed by radiation or removed by surgery.  For most moderate
grade cancers, it doesn't make too much difference whether it is organ
confined or not.

But to understand just what it means in the context of the Partin tables
requires some elaboration.  What Partin and his coworkers did was to
compare the presurgical diagnosis with what pathologists found on
examining the entire prostate following surgery.  It is not entirely
clear, but if you note that the percentages add up to essentially 100
percent, you can conclude that the categories are mutually exclusive.
if so, they are

1) cancer appeared to be confined to the prostate;

2) cancer extended beyond the prostate but none was found in the seminal
vesicles or lymph nodes

3) cancer was found in the seminal vesicles but not in the lymph nodes

4) cancer was found in the lymph nodes

The last two cases, particularly 4, suggest that there was a good chance
the cancer escaped the local area entirely and metastasized to distant
sites before the surgery.   Note however, that the pathologist has no
way of knowing whether the cancer has in fact metastasized.  Even if the
cancer appears to be organ confined, it is still possible it
metastasized, although the likelihood of that is pretty low in the great
majority of such cases.  And even if cancer is found in the lymph nodes,
it is by no means certain it has metastasized.