Silicon as an element is used by plants in a few different ways, but silica (silicon dioxide) is mainly used by plants to create “phytoliths”. What is a phytolith you ask?
“Phytoliths are rigid structures formed when dissolved silica in a plant's tissues accumulates in the cell walls or other cellular structures. As the plant dies and decomposes, the organic matter decays, but the phytoliths remain preserved in the soil or sediment.”
Essentially, they fortify a plant’s cell wall. This not only makes the stems of a plant stronger in wind, but fungi/bacteria can’t infect a cell that they can’t get into!
As for absorbing it in the first place, some plants have silicon transporters in roots that actively search for silicic acid in the soil and move it through the xylem. Silica will also diffuse directly into the soil/water slowly. The most well known silica accumulators are Bamboo, Sugarcane, Rice, Horsetail, and Cacti