A solid is something, which retains its shape.
It can be hard like iron or soft like rubber and fabric, flexible like plastic or stiff like wood, fragile like glass or tough like brick, it can be torn like paper, or remain strong like PVC, it may be malleable (squashable) like Playdoh but it will always keep the last shape you left it in and does not need to be kept in a container.
When the solid is a very small particle like sand or flour, it keeps its shape, but of course, we usually see lots of particles together. When there are lots of these together, they behave like a liquid, and we usually keep them in containers.