This is the beginning of a running gag in which Lucy builds up an incredible winning streak against Charlie Brown at checkers. It's a major part of the building of CB's defeatest attitude.
It is interesting to note Charlie Brown's reaction to his own behavior in panel 7. When he went on a rage tirade a few weeks ago, enough that the girls hid behind trees to get away (similar to Lucy's reaction here), we accepted it even though it'd look pretty disturbing in real life because comics exaggerate and illustrate emotions to enable us readers to more easily see them. Here, however, the comic takes his behavior and has him react to it with realistic dismay. It's a rather cool little deconstruction of the form.
