Well 'm assuming that by 'birth sign' you mean your Sun sign.
Firstly there's likely to be a time difference between the two. A Saturn return occurs approximately every 29 years and is the moment when Saturn returns to the zodiac placement it had when you were born. Using me as an example, Saturn was at 21 degrees Leo, so I experience a Saturn return each time that Saturn reaches 21 degrees Leo (I've had two of these) and the first of these occured when I was about 29.
When I was born the Sun was at 29 degrees Libra. So Saturn transiting my birth sign is when Saturn enters and tranists through Libra. That happened for me at age 3 for the first time and the next (and possibly last) time will begin at the end of this month. The transit can take about 2 to 2 and a half years.
Both are 29 year cycles but, unless Saturn was in the same sign as the Sun when you were born, the two occur at different points in your life.
The specific interpretation depends on what other planets are in the same sign as Saturn or Sun at birth, the dignity of Saturn in those signs and the houses in which natal Saturn and Sun lie.
For me the Saturn return occurs in my twelfth House though very close to the Ascendant. Leo is also the sign of Saturn's detriment, and Saturn has to conjunct Pluto and Mars before it reaches its return point.
My Sun sign transit is in Libra, which is the sign of Saturn's exaltation and occurs in my third house, which is more fortunate than the twelfth.
Usual interpretations treat Saturn as a restrictive influence, though it can also be constructive (structures are Saturn's domain). In some ways the first Saturn return can be seen as liberating, the structure has been completed and it's time to move on. If you take the Sun as a symbol for the 'self' then you can see your transit of your birth sign as giving some indication of how your personal development is going.
