Euclid was a Greek mathematician writer who lived around 300 BC. He wrote the most famous math textbook of history. He named his book ‘Elements’.
Elements continued to be used as a standard geometry textbook for 2000 years. Scientific geometry could not advance significantly during this period.
Geometry that is taught at secondary schools still bears the name of Euclid. It is called Euclidean Geometry. More advanced non-Euclidean geometries began much later during the 19th century.
Geometry is about shapes as algebra is about numbers. Shapes occupy space. Study of space is also a part of geometry. In his book Elements, Euclid covered shapes like point, line, circle and elementary solid shapes. His approach to these things is scientific and very impressive. It is believed that Euclid’s Elements is mostly a mention of the discoveries of earlier Greek mathematicians.
Abstract geometry has a very old history. Real math started with it. See a related article in this blog about the beginning of abstract math.
You may find it easier to think of shapes than numbers. Shapes are real images and not mere symbols.