Essentially, the "blue wave" means the Democrats have a majority in both chambers of Congress. They have broadcast widespread opposition to the MAGAgenda, and have the power to do so. Congress passes laws and confirms presidential appointees to executive offices. With Democrats in power, trumps agenda will no longer be rubber stamped by Congress, i.e. they have the power to refuse to confirm trumps nominees, pass laws to counteract executive orders, and control funding streams to rogue federal agencies. Unfortunately, there is not a blue veto-proof majority in Congress, so trump will likely veto a ton of legislation, but that's preferable to what the last two years have been.

Congress also has the power to open investigations into all the gross misconduct the administration has engaged in, as well as remove offenders from office.

Of course, it remains to be seen how much of these powers will be exercised, but we should remain hopeful.