The beatific vision is the vision of God, face to face, experienced by the angels and saints in heaven for all eternity. It is called the beatific vision because it is a vision that produces beatitude (or happiness).
In order to better understand the idea of the beatific vision, it is useful to understand other ideas first.
Human knowledge of things is limited by the means in which it is acquired. We learn everything through our senses. There is a famous expression that says: “there is nothing in the intellect that is not acquired first through the senses.” It is part of human nature to learn trough the senses because we are body and soul, material and immaterial, matter and spirit. Human beings are capable of knowing things in a spiritual way (in their essence) because we have immortal souls with an intellectual faculty capable of spiritual (immaterial) knowledge, but only by means of abstraction from the particular and material to the immaterial and universal.
We see a thing, for instance: an oak tree. It is a material and particular thing, an individual tree. Then our imagination (which is basically a picture making power) produces for us an image in our mind of the tree. The image is still material, because it can be seen and is still an individualized picture of the particular tree in question, but it is less individual than the tree outside our imagination because it is a copied image, not the complete original. Then, a faculty Aristotle called the possible intellect extracts the particular image of this tree and compares it to other known essences of its kind. So in this instance, other types of trees, e.g. fir trees, apple trees, palm trees, etcetera. From this, the faculty Aristotle calls the agent intellect extracts the abstract, immaterial, universal idea of the essence of what trees are by nature. When comparing all particular known trees in the possible intellect, the agent intellect is then able to grasp and extract what is essential to the nature of a tree in order to be a tree, and what is accidental about this oak tree or any other particular tree. The idea of “treeness” understood by the agent intellect is abstract and completely intellectual and immaterial (immaterial in this sense means not material, not unimportant). So now the person has an abstract idea of the universal essence common to all tress that makes this tree or any other fit the definition of what it is to be a tree. This idea isn’t the tree itself, but an abstraction from the oak tree. In this way, we know both the particular tree, and the essence or nature of all trees. The same process occurs for all abstract concepts human beings come to know, even things like unity, goodness, truth, and beauty.
It’s a pretty complicated process, and the knowledge we have of the particular tree, as well as the nature (or essence) of all trees is still limited. We don’t know the tree intimately in itself. There is an idea between the tree and us.
In the beatific vision, there is an immediate vision of the Divine essence without the aid of the senses or any intermediate idea. It is a way of knowing God that is more akin to the way that God knows things than the way human beings naturally know them.
God’s knowledge of a thing is different than our knowledge. God knows things in themselves, without intermediary ideas translating the thing known to the knower (in this case, God). So, God knows this particular oak tree, and it exists because God knows it. God’s knowledge of the tree is the tree.