Bingo corners are when you get the numbers in all 4 corners to make a bingo!
You will probably get this over time while grinding out the rest of the achievements.
But if you want this faster, you can either play with more cards at a time and hope for the best, or keep buying more balls with coins when the match is over till you get the corner matches.
Example of a bingo corner.
B I N G O
* o o o *
o o o o o
o o * o o
o o o o o
* o o o *