The Nether is an alternate world which is possible to enter via a Nether Portal. It is a vast and dismal hellscape occupied only by beasts the likes of which the overworld has never before seen.
Portals can be activated by anyone who is able to use flint & steel which includes donors of the Nether level or higher, admins, and moderators.
Be aware that the Nether can be regenerated at any time so anything constructed there will be wiped out.
Glowstone is a popular item to obtain in the nether, however Ghasts can make the task difficult.
Ghast tears have become a valuable commodity as ghasts do not spawn regularly, possibly due to low tps. Glowstone has also become rare in high-traffic areas due to the amount of people who have already mined most of it over the years.
Donor movement commands are available in the nether to those with the Nether Donation Package, or with the nether kit that can now be obtained with Tokens.