Among the talking heads, there are two primary memes regarding the meaning of today's elections. One is that this is a referendum on the president and his and the Democrats' agenda for the last two years. The other is that it is a values choice for the future. To some extent, both of these are true, which suggests, of course, that, perhaps to a greater extent, neither is true.
Going back to the founding, the story is that as America neared the decisive moment of declaring its independence from Britain, one-third of the population supported independence, one-third opposed, and one-third was neutral. If we use that breakdown as a model for the tipping point regarding American political revolution, and noting that only about 50% of the eligible electorate votes, that translates to a minimum success rate of two-thirds to declare an overwhelming action by voters. (Voting is an act, more than a "signal," or a "message.")
In most cases, the political direction sent by voters is typically only the statement of a little more than a quarter of the population (a majority of the half who bother to vote). To conclude something special has happened, more intensity is required. In other words, using the elections as a proxy for the public's true sentiment, don't bother talking about a voter rejection of the Democrats' agenda, or a mandate for a Republican agenda (if such a thing existed), unless there is an 80+ seat gain in the House, a new majority in the Senate, and two-thirds of the governors' mansions occupied by Republicans. Likewise, anything short of Democratic control of the House, a near filibuster-proof majority in the Senate, and the same two-thirds of the governor's mansions is less than a demand for the status quo and "staying the course." Barring something unexpected, neither set of three things will happen today.
The political act itself is rarely the catalyst for true sustainable change; culture is the engine of history, not politics and certainly not economics. Elections serve to reflect the current state and to give some guidance for what actions by the government are expected. Will today's results indicate a change in political momentum? Undoubtedly. A new path for the country? Perhaps. A sea-change in our nation? Probably not. For that, the political backlash needs to have taken root in something more substantial than an election cycle, and we really won't know until will see the fruit of that vine.
It is for this reason that the important question tomorrow will not be about "What it means," for there is limited clarity here beyond the obvious displeasure the electorate is expected to display, but instead "Where do we go from here?" And then it will be time to get back to work.