Minderwiz (or in any case someone more expert than me) can probably explain better,

but here goes.

We will consider perfect aspects because they are easier to understand than with orbs, moiety etc

An perfect aspect (say sextile) only means that the degree difference between the planets is exactly so many degrees (60 for sextile, 0 for conjunction, 120 for trine etc). It doesn't matter if one planet is 'before' or 'behind' the other.

At the moment of casting your chart, the Moon is at 2 degree Sagittarius. If there were a planet exactly 60 degrees away on *either side* , i.e at exactly 2 degrees Libra (2 degrees Sagittarius - 60 degrees) or (2 degrees Aquarius, i.e 2 degrees Sagittarius + 60 degrees) then the Moon would be forming a perfect sextile with them.

Now, think about this using Jupiter as a starting point. Jupiter is at 11 degrees Libra.

If there were a planet at exactly 60 degrees before him (11 degrees Libra + 60 degrees = 11 degrees Sagittarius) or behind* him (i.e at either 11 degrees Libra minus 60 degrees = 11 degrees Leo) , Jupiter would form a perfect sextile with that planet.

There are no planets at either degree. Yet.

But

The Moon is presently at 2 degrees Sagittarius, and *moving towards 11 degrees Sagittarius*(and is the fastest moving planet). When it reaches 11 degrees Sag, it will form the perfect sextile with Jupiter,because the distance between them will be exactly 60 degrees.

Since the movement of the faster moving planet (here Moon) is *towards* the forming of the perfect sextile, or, in other words, as time passes, the aspect gets closer and closer (Imagine the moon at 2 degrees, 5 degrees, then 10 degrees, then exactly 11 degrees Sag) the aspect is an "applying aspect".

After forming this exact sextile, the Moon then moves away, increasing the angle between itself and Jupiter (the difference become s65 degrees, then 70 etc) .This is called a 'separating aspect'. The exact aspect has already happened and the faster planet is moving away from exactness.

With retrogrades etc, this can get complex, but here Jupiter is not retrograde, and the Moon is never retrograde. So the above simple movements work.

Hope that helped