Many solid operations are performed on the currently active solid.
In a wireframe or shaded wireframe view, the wireframes of active and inactive solids are coloured differently; the colours depend on the colour scheme you use. If there is no active solid in your model, all solids are shown in their true colour.
To make a solid active:
To deactivate a solid, click its Active solid icon.
To activate a solid, click its Inactive solid icon.