Nord Stream 2 is a planned natural gas pipeline that will connect Russian gas supplies directly with the German market, underneath the Baltic Sea. It is a project of Gazprom, Russia's major oil company, and is planned to go into operation before the end of 2019. Gazprom is technically a private company, but the Russian government owns a majority stake; its leaders tend to have close ties with the government and, particularly, with Russian President Vladimir Putin. Russia is heavily dependent on oil and natural gas exports and trade with the European Union, making Gazprom politically important.
Germany, under Angela Merkel's leadership, wants the new pipeline, because it will help it to transition off nuclear power while still meeting emissions standards it has set for itself (natural gas is relatively clean-burning, as opposed to coal). As an added benefit, Germany would then act as a distributor to other European countries, enriching itself in the process. Russia wants the new pipeline, because it will allow its gas to bypass existing pipelines that flow through Ukraine, removing Ukraine from the revenue flow and further isolating the country. Russia has been steadily chipping away at Ukrainian sovereignty over the last few years, first with the annexation of the Crimea, then with the thinly-veiled Russian-backed rebellion in the eastern, ethnically Russian, part of the country. Hurting the country economically is part of the overall plan.
For these reasons, of course, Ukraine opposes the new pipeline as it would impact the revenue they gain from gas passing through their territory. In addition, many European politicians fear that the pipeline would make Europe even more dependent on Russian fuel supplies (thus, forcing Europe closer into Russia's orbit).
The United States is therefore opposed to the pipeline, because of its geopolitical implications; it would strengthen Russia's role in Europe while weakening the U.S.'s influence in the region. President Trump has expressed his opposition to Nord Stream 2 and is expected to take up this issue when he meets with Putin next week. Europe, for its part, is stuck in the middle.