I am not a geologist, but when I saw your question, I just had to find the answer--if only for myself.
Gabbro is defined as "a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass." I don't know about you, but this definition needs more explanation for me!
Igneous rocks are formed when magma or lava cools and solidifies; so gabbro is volcanic in origin. As the definition notes, gabbro is plutonic, meaning that the magma or lava from which it was formed got trapped under the earth before it cooled.
Gabbro is used commercially as a paving stone and, under the name black granite, as a source for grave markers.
I've attached a couple of articles that will give you more information.
A dark, coarse-grained plutonic rock of crystalline texture, consisting mainly of pyroxene, plagioclase feldspar, and often olivine