.xlsx files are files formatted for Microsoft's spreadsheet program, Excel (Wikipedia article). Specifically, .xlsx files are formatted for the current versions of Excel while older versions use .xls files. (Newer versions of Excel can still read the older .xls files, too.) Spreadsheet programs are designed to store data in tabular form, and often have the capability to perform statistical analysis. As a registered student, you have access to Excel, assuming the program is compatible with your computer. If Excel is not compatible with your computer you should be able to open a .xlsx file with Google Sheets, which is Google's spreadsheet program (Wikipedia article).
.csv is another tabular data file, but saved in a format that can be read by a wider range of computer applications. "csv" stands for comma separated value. A .csv file is essentially a text file where rows of data are separated by line breaks and columns are separated by commas (see example below). Unfortunately, .csv files have more limited capabilities compared to .xlsx and other spreadsheet specific files. .csv files can only be used to save values (numbers or characters), whereas .xlsx and other spreadsheet program specific files can also store formatting, calculations and other functions.
Header 1, Header2, Header 3, Header 4
1, 2, 3, 4
5, 6, 7, 8
9, 10, 11, 12