There are 4 diets - which gives 6 possible pairwise comparisons between them: 1-2, 1-3, 1-4, 2-3, 2-4, and 3-4 (2-1 would be the same as 1-2, so we don't count it twice).
The plot has all these comparisons displayed at different heights with the label on the left y axis side.
The x-axis represent the mean differences that were found between those pairs. So for example the comparison 2-1 had a mean difference of 20.
The extended lines show the 95% confidence intervals. In this case if the confidence interval crosses the 0 point - the difference would not be statistically significant. In the plot colors seem to indicate this significance: red lines do not cross the 0 so the differences in their means were found to be statistically significant.
Based on this we could reason that diet No. 1 was different from the rest as all the pairwise comparisons that included diet 1 were statistically significant (red in the plot) while all the other comparisons were not.