Piet Mondrian is considered one of the pioneers of abstract art and is best known for his grid-based compositions featuring rectangles of red, blue, yellow, and black. The meaning behind his work is rooted in his philosophical and spiritual beliefs, which he called "neoplasticism."

For Mondrian, abstraction was a way to express universal principles and to create a sense of harmony and order. He believed that abstraction could provide a window into the spiritual realm and that his paintings could help to create a new, more harmonious world.

Mondrian's use of basic geometric shapes, such as squares and rectangles, is meant to symbolize the fundamental building blocks of the universe. The red, blue, yellow, and black colors he used are meant to represent the primary colors, which he believed were the most basic and universal of all colors.

The grid-like compositions of Mondrian's paintings were meant to evoke the feeling of stability and balance, and to suggest that everything in the universe was connected by an underlying structure. By simplifying his compositions down to the most basic elements, Mondrian aimed to create a sense of clarity and to reveal the underlying structure of the universe.

In summary, the meaning behind abstract art by Piet Mondrian is rooted in his philosophical and spiritual beliefs, and is meant to evoke a sense of harmony and order through the use of basic geometric shapes and primary colors. By simplifying his compositions, Mondrian aimed to create a sense of clarity and to reveal the underlying structure of the universe.