1. The green vertical line represents the sample average estimated from the data. "True" mean is initially set to this value.
3. The distribution of the test statistic (i.e., one-sample t test) under the 'True" mean has a non-central t-distribution. When you move the "True" mean close to or away from the hypothesized mean, the probablity density function changes. The following figure shows the non-central t-distribution with different values of non-central parameter μ, and ν degree of freedom Noncentral t-distribution - Wikipedia, the free encyclopedia
4. The red represents t-distribution.
5. The blue represents non-central t-distribution.