An interaction occurs when the response is different depending on the settings of two factors. They will appear with two non-parallel lines, indicating that the effect of one factor depends on the level of the other.
For a more general set of graph tools see Model Graphs - General Information
For factorial analysis, the I-beams around the predictions are the result of least significant difference (LSD) calculations. The LSD I-beams are approximate tests to check if the predictions at the displayed factor combinations are significantly different. Click on a prediction symbol to draw the graphs with individual LSD bars for pairwise comparisons. Click any where else on the graph to bring back the I-beams.