This means that we will be filled with great joy whenever we find ourselves wanting to be good, do good, and see good in the world. It is such a strong feeling, that we will do almost anything to see that it happens.
“Righteousness” is the habit of thinking and doing all that is right and good. This Beatitude does not say we will be blessed, or happy, when we have become righteous, but that we are blessed because of our strong desire for righteousness. That strong desire will make us do what is necessary to see that we do what is right, and that others are treated justly.
To “be filled” is to have a feeling of fullness, or to know for sure that you have everything you need. For instance, after eating a really good meal, your tummy is no longer empty, but “full.” This Beatitude tells us that if we continue to “hunger and thirst” for righteousness – all that is right, and good, and just – we will be filled with the ability to do good.
However, we do not get that good feeling of fullness just so that we can sit back and enjoy our own personal comfort. No! We have to do what we know to be true to help others.