When someone asks you for something in "tabular format" they're just asking for it to be arranged in a table. This means putting data into a series of rows and columns. For example, when you input data into an Excel spreadsheet, you're putting it in tabular format. Many programs give you the ability to alter the appearance of your table by adjusting features like their background, borders, and fonts. This can make the data that you're presenting easier to read and understand.
Data tables are used in a variety of fields. They're popular because they offer an easy way to present large amounts of hard data. They also make it easier for viewers to make comparisons between different data sets. For example, you might be asked by your boss to put a list of political donors into tabular format. This would entail making columns for all of the most important data and then rows for each separate contributor. After doing this, you could another row that references when each person was last contacted or one referencing how much they've already contributed. Armed with this information, a political campaign will be able to make smarter decisions about who it reaches out to for funds.
The next time someone asks you for something in "tabular format" all you need to do is put the information in a table. You can see what tabular format looks like by examining a typical Excel spreadsheet.