Geckoboard widgets are constantly refreshing to keep your data fresh - in other words, to display the most recent value for the metrics they enclose.
It is possible though that at a give point in time the most recent value can't yet be calculated.
Imagine that a widget is set up to display a leaderboard with top ticket solvers today, but nobody in the team has solved a ticket today yet. It would be inaccurate to stretch the period to have something to show and displaying a 0 (zero) might be a bit confusing as the visualization in question is not a number but a leaderboard that perform some calculations on your data.
In cases like the above example, we will display "no data to visualize", as there isn't yet any data that the widget can use to build its visualization.
Needless to say, this is a temporary state. As soon as the widget successfully gathers the data required to build its visualization, this will be displayed.
If you see "no data to visualize" on a widget often, you might want to consider adding filters to better control its output, or changing the metric's time-period.