Yin and yang are the two forces the I Ching uses to describe any situation. Yang pushes, starts, moves and asserts. Yin receives, holds, adapts and gives shape. They aren't good and bad. Each needs the other, and each carries the seed of the other inside it.
Where the words come from
Originally, yin was the shady slope of a hill and yang the sunny one. Over time the words came to name all kinds of pairs: cold and heat, stillness and motion, night and day.
An honest footnote: the oldest layer of the book, the Zhouyi, hardly uses these words at all. There the lines are named by numbers, nine and six. It was the later commentaries, the Ten Wings, that read the whole system in terms of yin and yang.
In a reading
Every line is either yang (a solid yang line) or yin (a broken yin line). Six of them make a hexagram. When one force reaches its extreme, it turns into the other, and that is a moving line.
For example, Activity has six yang lines and Passivity has six yin lines. Most of your casts mix the two, and the balance between them says a lot about the moment.
To go deeper, read Yin and Yang.