A living organism that produces its own food through photosynthesis, has roots, stems, and leaves, and grows from seeds or vegetative propagation.
Plants are organisms that capture sunlight and convert it to food. They have roots (absorb water/nutrients), stems (transport water/food), and leaves (photosynthesis). They reproduce via seed or vegetative propagation. Understanding basic plant biology helps you grow them successfully.
Understanding what plants need (light, water, nutrients, air) helps you provide proper conditions and diagnose problems.