In any astrological aspect, the applying planet is the one which, if moved forward in the zodiac, will make the aspect exact. The separating planet is the one, which, if moved forward will make the orb of the aspect greater.
I think of this as kind of like the roles of male and female in traditional society, where the male is supposed to be the one to ask the female for her phone number, or for a date. The male is supposed to PURSUE — like the applying planet in an aspect, and the female ELUDES, like the separating planet, which stimulates the male’s chasing. She RECEIVES his energy.
The energy in an aspect flows from the applying to the separating planet. That is, the applying planet is ACTING ON the separating one. The applying planet INITIATES the action. For instance, I have a Mercury/Uranus conjunction. Mercury is at 20 degrees Gemini and Uranus is at 25 degrees Gemini, so Mercury applies, and Uranus separates. I have a thought — Mercury is thinking — and this initiates the connection to Uranus. Then the thought LEADS TO something unexpected, a surprise, a discovery, something unique — Uranus. In other words, the Mercury energy comes first, and then triggers Uranus.
Now if it was the other way around, Uranus at 20 Gemini, and Mercury at 25 Gemini, then Uranus would apply and Mercury separate. That would mean, say, that I have the urge for freedom (Uranus), and this would lead me to think about it, to analyze how I might get it, use my cleverness (Mercury) to obtain that freedom, or check out connections (Mercury rules connecting) that might bring me freedom.
The applying/separating designation offers another layer of subtlety in analyzing aspects. It’s especially important when a planet in one chart aspects a planet in another. In this situation the person with the applying planet tends to be the one who needs to initiate the activity that is to occur between themself and another person. If the person with the separating planet tries to initiate the connection, it may not work, may not feel right, may not get anywhere.
This becomes especially acute when several of a person’s planets have applying aspects to the planets of another’s chart, without the second person having any applying planets to the first person’s chart. This can create a situation where the person with all the applying planets is holding all the cards, so to speak, and hence must be the one to initiate most of the activities between themself and the other person.
Remember that this designation of applying and separating can be applied to ALL ASTROLOGICAL ASPECTS, and the more you get used to using it, the easier it is to understand.
And by the way, I got this concept from Marc Edmund Jones’ book “The Essentials of Astrological Analysis,” a tome which I’ve often told people I might one day translate into English (that’s a joke because the book is written in English — just not the kind that most people speak!)