Sea levels refer to the average height of the ocean's surface, measured between high and low tides. They can change due to various factors, including the melting of ice from glaciers and polar ice caps, as well as thermal expansion of seawater as it warms. Rising sea levels can lead to coastal flooding and erosion, impacting ecosystems and human settlements. Monitoring sea levels is crucial for understanding climate change and its effects on the environment, as well as for planning and protecting coastal areas from potential hazards.