Recurring pro-Kremlin disinformation that is aimed at justifying Russia's annexation of the Crimean peninsula in February 2014.
The mentioned Budapest Memorandum is a diplomatic document that was signed in December 1994 by Ukraine, Russia, the United States and the United Kingdom. Under the Memorandum, Ukraine agreed to remove all Soviet-era nuclear weapons. In return, the US, the UK and Russia signed the Memorandum with guarantees of the territorial integrity and sovereignty of Ukraine. According to the document, all signatory countries have to respect Ukraine's sovereign rights for its territories.
Notable is the second paragraph of the Memorandum which states that the signatories "reaffirm their obligation to refrain from the threat or use of force against the territorial integrity or political independence of Ukraine, and that none of their weapons will ever be used against Ukraine." That is, through the Budapest Memorandum, Russia pledged not to use any weapons against Ukraine.
In this regard, Russia's illegal annexation of Crimea in 2014 and the Kremlin's support towards separatists in eastern Ukraine are a clear violation of the international memorandum.
Pro-Kremlin media outlets have repeatedly tried to downgrade the significance of the Budapest Memorandum. See an earlier case here, claiming that the document is merely a political statement.