Here's a breakdown:

* Hardened magma: This refers to igneous rock, which is formed when magma cools and solidifies.

* Squeezed into horizontal spaces between rocks: This describes the way a sill forms. Magma, being less dense than surrounding rock, will often find its way into existing cracks and fractures. If it intrudes horizontally between layers of rock, it's called a sill.

Key Features of a Sill:

* Horizontal or nearly horizontal: They typically run parallel to the existing layers of rock.

* Sheet-like shape: Sills are usually relatively thin compared to their lateral extent.

* Igneous composition: Made of solidified magma, like granite, basalt, or other igneous rock types.

Let me know if you want to learn more about different types of igneous intrusions, like dikes or laccoliths!