Physically retrogrades are when a planet appears to be moving backwards through the sky. Though all planets, and asteroids only move in one direction, from Earth, because we are all moving at different speeds planets sometimes appear to go backwards.
All planets and asteroids go through a retrograde period, Mercury generally goes retrograde for a few weeks 3 - 4 times per year. The outer planets and asteroids generally go retrograde for one several month period once a year. The slowest moving planets being in retrograde the most often. Pluto is in retrograde about 5 months out of the year. Venus generally is retrograde every two years. This is an over simplification, but will help for some ballpark information.
When going retrograde each planet will go over the degrees of the zodiac it has just been through.
For example this current (Jan 10, 2022) Venus retrograde started on Dec 19, 2021 at 26° Capricorn and will go direct at 11° Capricorn on Jan 29, 2022. However to understand the impact of any retrograde you have to go back to when it first hit the degree it will eventually go direct at.
Continuing with this Venus retrograde as an example. This process began on November 17, 2021, when Venus first went by 11° Capricorn.
Energetically what is happening is we are doing something in the initial transit, re-examining it during the retrograde, then finally redoing it as it goes back over those degrees for a third time.
This re-examining during the retro phase is what sometimes makes us feel stuck as we get a closer look at things that aren't working for us. With Venus this means we get an opportunity to look closer at struggles in our relationships. You'll want to look at which house this is happening in for you to get the most our of understanding it. Often it will cross 2 signs and houses, you will also want to look to see what planets in your chart it makes a connection to. Especially at either end of the retrograde. So again with this Venus retrograde any thing close to 11 or 26 degrees Capricorn are going be the most influenced.
The reason the ends are so important is that the planets appear to slow down as they stop and turn around so we will be hyper-focused as the planet or asteroid stations to move retrograde or stations to move direct.
You will also notice that with Uranus, Neptune & Pluto we are almost always somewhere in the retrograde zone. This constantly going back and forth over the same area, taking 2 steps forward and 1 step back is part of the reason they take so long to move around the zodiac. It is also why these planets are transformational in whatever area they are influencing as we sit with them for a while integrating and fully processing. Mercury, Venus, and Mars have less of a long term impact and are more about tweaks in our everyday lives.