The Term "vibrational alignment" simply means that you need to be a match to the frequency of your desire.

Here are some examples:

You have a desire to have $5,000 and the universe instantly grants the wish of $5,000. That money exists right this very minute in a parallel reality that you can not yet see or feel or touch from your asking for it. Whenever you ask for something it is instantly given.

It's similar to how we cannot see radio waves or cell phone signals travelling through the air in front of us. They are most definitely there, but we are just not allowing ourselves to tune into seeing those particular frequency waves.

So you launch this desire (that is in existence right now) but you are not sure if it will actually come. Or you are in the mindset of lack or possibly the feeling of unworthiness. Your conscious mind may think that you are open to receiving the money, but your chronic and practiced vibration is set to a different or lower frequency.

Knowing with zero doubts that you have $5,000 right now = High frequency = Vibrationally Aligned

Being unsure of your desire or feeling unworthy = Lower frequency = Not a Vibrational Match to your desire

So if you can start to believe with no doubts that what you ask for is instantly given to you, waiting for you to come into alignment with the frequency of it's existence even if you cannot see it with you eyes and physical senses, then your desire will eventually manifest into your reality. It must manifest because that is the law of the universe.

Does it mean just to be happy? Does it mean to let go?

These are two ways that you can become aligned, yes.

If your desire makes you happy and feels good, and you stay in that happy state consistently with zero contradicting thoughts, then you are in alignment with that desire.

The same goes for the idea of letting go. If you are 100% confident that what you want is on it's way and know it without a shadow of a doubt, you can then just stop thinking about it completely. Being confident and not thinking about it will release any resistance (or non vibrationally aligned) and you will essentially be allowing it to come into your physical existence through the alignment matching up.

Think a desire - know it exists - allow it to come - alignment is done :-)