Line is the second primitive term defined by Euclid.
To build a concept, we need to have some assumptions.
Based on that assumptions only, we can prove other theorems and lemma.
According to the assumption, Eulcid defines line as a single dimension and hence it is a continuous extent of length without breadth.
Subsequently, surface is a two dimension figure, with length and breadth.
And solid is a three dimensional object, with dimensions, length, breadth and height.