An Essay On Man is a philosophical and theological work in which Pope seeks to explore the relationship of God to Man. Most specifically it is a warning that mankind is not the centre of all things but must take heed of God's mysteries in order to bring about its own salvation.
The fourth segment of the essay explores the question of what makes us happy. He posits that since God wants to give all of humanity the potential to be happy, it must therefore be social rather than individual. Also that cooperation is a sure route to social happiness.
This is the meaing of "true self-love and social are the same": that if we really want to love ourselves, and to bring ourselves the greatest amount of happiness, that is best pursued by trying to bring happiness to society as a whole. We will be less happy as individuals if continue to see and deal with unhappiness in others.
This section of the essay also seeks to define happiness. Pope argues that it can't come from material things or fame and indeed that accumulating these things can be deleterious to happiness. Rather he equates happiness to "virtue" which he says:
Lies in three words, Health, Peace, and Competence
And this is the meaning of "virtue only makes our bliss below", that we must seek happiness in the pursuit of virtuous deeds. "Below" here is a theological reference to life on earth "below" heaven, the latter being a perfect state of grace where virtue and happiness are a given. Pope has perhaps chosen this shorthand to keep with his rhyming scheme.
The final line "all our knowledge is, ourselves to know" also seems to be a theological reference. We cannot truly know God: the ways of God are mysterious and we can only trust that they are good and just. Instead, rather than seeking to unravel the question of faith we need to concentrate on the good - the virtue - that we can do here on earth in order to spread happiness.
These final lines are thus essentially a summary of the arguments he's been making through this section of the essay. Broadly that people must use their faculties - their "reason" and "passion" - to live virtuously and thus spread happiness on earth without worrying about their eternal reward in heaven.