The overall mean is the mean of all observations, as opposed to the mean of individual groups. In ANOVA, for example, this is the mean of all observations across factor levels, as opposed to the means of individual levels.
Here is a main effects plot comparing absenteeism at each of four different local schools. Each dot on the plot identifies the mean absenteeism of students from a different school. The line drawn across the plot (called the center line) marks the overall mean: the mean absenteeism of students from all schools.