Fertilization in the case of plants refers to two separate concepts. It occurs when a pollinated plant reproduces itself. Fertilization can also refer to the act of adding outside nutrients to the soil around the plant or directly to the plant itself.
When a plant is pollinated, genes from the male flower are brought to the female flower, usually carried by insects or the wind. The sperm from the pollen enters the egg inside of the female flower's ovary. The sperm fertilizes the egg, which then forms a seed or a fruit with a seed at its center. A new plant grows from the seed.
The other kind of fertilization is when fertilizer is added to the soil around a plant. This is also called feeding the plant. According to Sunset Magazine, plants require three major nutrients. These are nitrogen, phosphorus and potassium. These elements can get depleted from the soil over time, usually when the land is used heavily, as in farming. When this happens, the nutrients need to be replaced. This is done by adding either chemical or organic fertilizers. Nitrogen can also be added by planting cover crops of special plants that are able to fix nitrogen from the atmosphere into the soil. Plants such as seaweed tend to be more popular for organic fertilizers.