The t-distribution is used for estimation and hypothesis testing of a population mean (average).
The t-distribution is adjusted for the extra uncertainty of estimating the mean.
If the sample is small, the t-distribution is wider. If the sample is big, the t-distribution is narrower.
The bigger the sample size is, the closer the t-distribution gets to the standard normal distribution.
Below is a graph of a few different t-distributions.
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