Articles in English are : a , an and the.
the first two "a", and "an" are used before nouns to indicate a single object or instance. For example one tree - a tree, one apple - an apple. These two articles are used when we are referring to any object of that type tree or apple.
When we want to refer to a particular object, or event or instance , that we already are familiar with, then we use the article "the".
I have an apple in my hand. I will eat THE apple.
I see a tree some distance away. I want to climb THE tree.
Here, the apple and the tree refer to those nouns mentioned in the earlier sentence.