Why is this important?
This is the first time a person close to President Trump has said that it was a "quid pro quo", ie "something for something", which is what the Democrats are claiming in the judicial process.
President Trump has consistently denied that it was a requirement of hand administration to investigate Joe Biden's family, but now a person who is close to the president is contradicting what he says in an sworn testimony.
What is the difference between this witness and previous witnesses?
When the first whistleblower emerged, President Trump and his supporters dismissed it as secondhand. When a second whistleblower, who heard the call himself, emerged instead criticized that he was anonymous and that Trump could not meet his accuser. Then came new witnesses, a Ukraine expert and an Eastern European expert who both worked for the Trump administration, as well as a man hand-picked by Trump's own Foreign Minister Mike Pompeo as deputy ambassador to Ukraine, and gave similar testimony, but all three testimonials were dismissed by Trump and his supporters saying they were "Never Trumpers," and the president tweeted words like "human garbage."
But now it is a Trump supporter, Sondland, who gave large sums to the president's campaign, and who was rewarded with a nice ambassador post, which says the same thing. This time, he cannot be dismissed as neither anonymous, Trump opponent, nor anyone with secondary information, because this is a person with regular contact with Donald Trump.
Why does Sondland make this extension?
It is hard to know exactly, but several people have testified under oath after Sondland and provided information that did not match his first story, which freed Trump. It is illegal to lie under oath, it can give severe punishment. If Gordon Sondland had come to lie, it would cost him dearly.
What happens now?
The hearings around the national court continue. When, or if, the Democrats think they have enough on their feet, they will formulate the formal suspicions that exist against the president, so-called "articles of impeachment," and after that vote the House of Representatives around whether the president should be brought before state law.
In the House of Representatives, Democrats are in the majority, so it is relatively likely to happen. Then it goes to the Senate, which holds the process of national law itself. It requires a two-thirds majority to dismiss the president. That has never happened in US history, and since the Republicans are in the Senate majority, it will take a lot for that to happen now.