The Part of fortune is calculated based on the positions of the Sun, Moon and the Ascendant in combination. There has however remained some confusion about this. The Arabic and Hellenistic way of determining the position of the Fortuna is the same as in later western times (by interpreters like William Lily) but with one major difference. If the birth chart is a night chart (sun below the horizon), the relation between the sun and the moon will be reversed, whereas in a day chart it is the same as the Lily formula. What this means in a practical sense is that a night-birth chart according to the older technique would have its Lot of fortune exactly opposite its expected place in the western interpretation.
What does it signify? Again, interpreters have differed from ancient times till modern. In general the idea is that the part of fortune, that is its exact degree placement in your horoscope, represent a point of success, good fortune, insight and prosperity. According to some it has a connection to the ascendant through its association with the physical body and health state. Investigating the Sabian symbol of the degree of your Part of fortune can be an interesting exercise!
In more general terms, the area (not only degree) of your Part of fortune may represent a sphere of mind, or of life that you feel develops naturally and without much effort. One of the few where luck actually seems to accompany you!