The equator is an imaginary circle that divides our planet into the Northern Hemisphere and the Southern Hemisphere. It is not a real line on the ground - but geographers and map-makers have calculated exactly where it runs. At the monument on São Tomé, you can put one foot in the north and one foot in the south at the same moment.
Countries and places on or very close to the equator get almost exactly 12 hours of daylight every day of the year. That is very different from countries further north or south, where days are long in summer and short in winter. On São Tomé, sunrise and sunset happen at roughly the same time all year round.
The sun is also very strong near the equator because it shines almost directly downward rather than at an angle. This is why equatorial countries like São Tomé and Príncipe are warm all year, with lots of rain that feeds their thick rainforests.
The monument itself is a small white marker beside the road, and it is popular with visitors who want a photograph of themselves with one foot in each hemisphere. Schoolchildren from the island often visit as part of their geography lessons.