Congress Questions All The President's Men

Legislators have called these members of team Trump to testify in recent weeks.
Who Has Been Called To Testify?
Former FBI Director James Comey and Attorney General Jeff Sessions were perhaps the highest-profile figures to testify on Capitol Hill, but congressional lawmakers have hosted numerous public and private meetings on the Russia investigation. They've questioned intelligence experts, current and former government officials, and Trump associates. The House intelligence committee's witness list alone reportedly included "3-4 dozen names."

Other Trump associates, including Donald Trump Jr. and Paul Manafort, have provided documents and other records detailing their contacts or other business dealings with Russian officials.
What Have We Learned?
Comey's public testimony revealed details about his private meeting with Trump. According to Comey, the president asked the FBI director for a loyalty pledge, and told him to drop the investigation into Michael Flynn. Responding to questions from the Senate Intelligence Committee in June, Comey explained that he'd taken detailed memos about his contacts with Trump due to concerns the president "might lie about" them.

We also learned that Sessions doesn't recall very much about his meetings with Russian officials. In May, then-acting Attorney General Sally Yates revealed that she'd warned Trump that Flynn might be compromised.
Why Does It Matter?
Much of this testimony happens under oath, meaning there could be significant consequences for lying -- perjury is a felony offense. Beyond what we know publicly, closed hearings have presumably also provided the House and Senate intelligence committees with evidence to pursue their investigations, which will culminate in a pair of reports describing their findings. 

Some have questioned whether the Republican Party-controlled panels can be trusted to vigorously and impartially follow the facts of the investigations.
How Did Trump Respond?
Loudly, and occasionally in real time. Have you not heard? There's absolutely nothing to the Russia story, so everyone should just stop looking already.