In Greek mythology, Boreas was the god of the north wind. From that, boreal has come to mean “of the north” or “northern”.
Boreal is not really associated with “of the northern hemisphere” in quite the same way as “austral” has come to mean “of the southern hemisphere”. But when, for example, we need to talk about “summer in the northern hemisphere” in contrast with “summer in the southern hemisphere” we sometimes talk of the “boreal summer” and the “austral summer”.
The word that most frequently follows boreal is forest. The boreal forest is also called the taiga, a climate zone with long, cold, snowy winters, where the dominant trees are conifers: larch, spruce and pine. Mainly in Scandinavia the boreal forests also have a component of birches, a deciduous tree that loses its leaves in winter. At the northern edge of the boreal forest/taiga zone, the summer growing season gets shorter than farther south, and the trees get shorter and sparser. This landscape is the ecotone known as forest-tundra. As you travel farther north, the trees fade away completely, and you enter the tundra zone.