A bluebell wood is a woodland that in spring-time has a carpet of bluebells (Hyacinthoides non-scripta)

Common Bluebell

Hyacinthoides non-scripta, commonly known as the common bluebell, is a spring-flowering bulbous perennial plant. -Taxonomy:...

 underneath a newly forming leaf

Leaf

A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....

 canopy. The thicker the summer canopy, the more competitive ground-cover is suppressed, encouraging a dense carpet of bluebells, whose leaves mature and die down by early summer.

Bluebell woods may be found in all parts of Great Britain

Great Britain

Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

 and Ireland

Ireland

Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...

, as well as elsewhere in Europe. Bluebells are a common indicator species

Indicator species

An indicator species is any biological species that defines a trait or characteristic of the environment. For example, a species may delineate an ecoregion or indicate an environmental condition such as a disease outbreak, pollution, species competition or climate change...

 for ancient woodland

Ancient woodland

Ancient woodland is a term used in the United Kingdom to refer specifically to woodland that has existed continuously since 1600 or before in England and Wales . Before those dates, planting of new woodland was uncommon, so a wood present in 1600 was likely to have developed naturally...

s, so bluebell woods are likely to date back to at least 1600.
Gerard Manley Hopkins

Gerard Manley Hopkins

Gerard Manley Hopkins, S.J. was an English poet, Roman Catholic convert, and Jesuit priest, whose posthumous 20th-century fame established him among the leading Victorian poets...

, one of the romantic poets, was very keen on the plant as revealed by these lines of his poem "May Magnificat"

And azuring-over greybell makes

Wood banks and brakes wash wet like lakes

In his journal entry for 9 May 1871 Hopkins says:

In the little wood opposite the light they stood in blackish spreads or sheddings like spots on a snake. The heads are then like thongs and solemn in grain and grape-colour. But in the clough through the light they come in falls of sky-colour washing the brows and slacks of the ground with vein-blue, thickening at the double, vertical themselves and the young grass and brake-fern combed vertical, but the brake struck the upright of all this with winged transomes. It was a lovely sight. - The bluebells in your hand baffle you with their inscape, made to every sense. If you draw your fingers through them they are lodged and struggle with a shock of wet heads; the long stalks rub and click and flatten to a fan on one another like your fingers themselves would when you passed the palms hard across one another, making a brittle rub and jostle like the noise of a hurdle strained by leaning against; then there is the faint honey smell and in the mouth the sweet gum when you bite them.