How to | Work with Nested Lists
Nested lists are lists within a list; they are the principal structure for data in the Wolfram Language and allow for high-dimension arrays and ragged datasets as well as common uses such as matrices.
Use [], the short form of the Part function, to get the second row:
Use Flatten to remove the nesting:
Mean now lists the means by each individual nested sublist:
Use Flatten to obtain the mean of the entire dataset: