Happy Solar Return!! This is an astrologer's way of wishing you a Happy Birthday! Around your birthday (plus or minus a day or so), the Sun returns to the precise place it was when you were born. Hence, it is called a Solar Return (return of the Sun).
When you know the accurate time of your birth, an astrologer can calculate a chart for the precise moment that the Sun returns to its natal (birth) position. It is important to have an accurate birth time, since the margin of error in the Solar Return chart is very sensitive to the birth time accuracy. As a general rule, the Solar Return chart can shed light on what kinds of events and themes are likely during the coming 12 months.
There are actually two different ways to calculate a Solar Return. The most common one is to calculate when the Sun returns to the same Zodiac position that it has in your Natal chart; in other words, to the same degree and sign it was in when you were born.
The other way is to calculate the precise spatial position relative to the fixed stars. This takes into account the Precession of the Equinoxes. This is called the Precessed Return.
The difference between the Zodiac position and the spatial position is very small, but is considered significant by many astrologers. This is because it can dramatically change the Ascendant and house placements in the Solar Return chart. Also, the Moon is often in a different sign and makes very different aspects in the chart.
The date when your Precessed Solar Return takes place is is a bit later than your regular Solar Return. The older you get, the longer you'll have to wait after your birth date to reach your Precessed Solar Return. For example, on your 20th birthday, the Precessed Return takes place about 6.5 hours after the regular Return. On your 40th, there's a difference of about 13.5 hours. When you turn 60, it will be about 20 hours. And when you turn 80, you'll have to wait about 26.5 hours to reach your Precessed Return.
In my experience, the difference between the regular and the Precessed Solar Return charts is one of perspective. The regular chart tends to show what you hope will happen, what you're aiming for and what your subjective experience of the coming year will be. The Precessed chart tends to show the objective reality, the external conditions and circumstances you are likely to encounter in the coming year.
(Note: A Solar Return has nothing to do with retrogrades, since the Sun never goes retrograde.)