What does it mean for a molecule to have a definite shape???

When we say something has a definite shape, it means:

It keeps it's shape and does not change unless an external force is applied.

In other words:

The object does not flow or spread out to take the shape of its container.

Its shape stays constant over time.

Examples:

Solids (e.g rock, book) has a definite shape because it keeps its shape unless broken or cut.

Liquid (e.g., water, oil) has no definite shape cos it flows and take the shape of any container.

So its shape varies. It is not constant.

Gas (e.g, air) has no definite shape because it expands to fill any container and have no fixed shape.

So in science (especially physics or chemistry), saying a material has a definite shape usually means it is a solid.