You can find here a good definition and tutorial on what is pairwise testing:
In two words, the idea is trying in a combinatorial way all possible pairs of inputs to the program, or if not possible, the maximum possible providing a maximal coverage of the tests.
So, in a sense, you may consider the used browser as an input to the program (not very standard, but hey, why not...), and test the different possible combinations. Here is a very simplified axample, inspired from the example from the above link: You have a web application to order a car transaction (buy or sell), and you want to try the different possible browsers: IE, Firefox, Chrome, Edge, Safari, Opera. So you have 2 possibilities for the transaction type (buy or sell ), and 6 possibilities for the browser, so you have to make 12 tests for each possible combination.