The objects which have definite shape,distinct boundaries and a fixed volume is called as solid objects or they are in solid state.
A book, pen and pencil,
A car or all the vehicles, etc.
All the objects which have definite shape are come in to the category of solid.
The solid objects are tend to stay on their original shape despite the external force is applied.
For example if we applied external force to the chalk piece it will broke into pieces but the shape and structure of the particles will not altered. because solids are rigid bodies.
Remember that the molecules inside the solids are so condensed and this is the reason that they are rigid and maintain the definite shape.
The inter molecule force in the solid is the strongest force compare to the liquid and gas. This strongest inter molecule force makes the solid matter denser and gives a certain shape and structure.
Observe the above figure, and you can see that the molecules inside the solid are dense, that's why solid objects are so rigid.