What happened?
On 6 June 2019, the Acting US Defence Secretary Patrick Shanahan wrote to his Turkish counterpart, Hulusi Akar serving Ankara an ultimatum demanding its withdrawal from a defence deal with Russia for the acquisition of S-400 ‘Triumf’ anti-aircraft weapon system.
According to the letter from Shanahan. Turkey is also under the threat of facing sanctions under the Countering America’s Adversaries Through Sanctions Act of 2017(CAATSA) with any country engaging with Russia’s defence sector.

The Turkish response was that it was too late to pull out of the deal with Russia.

What is the background?

S-400 Triumf is an anti-aircraft defence system with a range of up to 380Km, described by The Economist in 2017 as "one of the best air-defence systems currently made".

US and Turkey have been having a row over the S-400 deal for some months. The repeated US warnings did not stop Turkey from proceeding steadily. The American-made F-35 fighter jets are only the beginning of the threats as there is a strong bipartisan determination in the Congress to impose sanctions. In March, Curtis Scaparrotti a US Army general and NATO Commander had warned of cancelling the F-35 program. 

The US fears the S-400 systems in Turkey might provide Russia with intelligence regarding the F-35, 100 of which Ankara just purchased. The concern is a little excessive as there are F35s already in operation near China. 

From the Cold War era to US support to Kurds, the countries have not had a harmonious past.

Turkey has been leaning towards Russia and heading away from democracy following the current president Recep Tayyip Erdogan amassed control of the state’s military and budget. The demand for the S-400 comes from the need to protect the President from stray pilots attempting to depose Erdogan, the lack of which is a weakness.
Turkey surprised everyone by declining the US offer to buy ‘Patriot’ systems and going with Moscow.

What does it mean?

Shanahan has made it explicit that the pilot training program will end on July 31 by when few but not all trainees would be able to finish the course. This seems to have made no effect on the deal as Turkey is seemingly in need of defending rather than attacking currently.

A Moscow-Ankara bridge is on the rise. Heads of the state have met each other in person seven times in 2018 and spoke 18 times on the phone. America is worried about the endearment but intends to choke Ankara since 937 parts of F35 are manufactured by Turkish companies contributing around $12 billion to the economy.

Turkey seems unrelenting and the US is vying for the upper hand. 
The US is concerned about Russia improving ties with several nations and has made similar threats and actions against other countries dealing with the Russian defence including India, Iran and North Korea. Turkey is adopting an increasingly independent defence strategy and distancing from the US seems to be an important step.