A table is just a basic way of organizing data into rows and columns. If you have some information like high temperature for each day of the year your table might look like this:
DAY High Temperature (ºC)
January 1 2
January 2 -5
January 3 -7
You could write this out in text like this: "On January first the high temperature was 2 degrees celsius. On January second the high temperature was seven degrees colder...." The problem with this is that it makes it very difficult to understand the information and takes a lot of text. A well organized table allows you to quickly scan through a set of data and see simple trends. It also stores information very efficiently and makes it easy to update without making errors.
If you're doing simple graphing for a math class, the headings of the columns will be more boring things like X and Y.