It's a table. Think of a table from a database or a 2-dimensional csv file that exists as an object in your programming language of choice. The exact implementation is different in different languages and libraries. In Python the most popular one is the DataFrame class from the pandas library, R has data.frame built in, Julia has DataFrames.jl and there are other implementations too - spark, dask, data.table, tibble etc.

The easiest way for this is to click is open up Python or R and play around with a dataframe, it really isn't that complicated.