For example, think about 8.4 - 2.9.
You have to regroup because you can't subract 4 - 9.
4 turns into a 14 since you borrow from the left number, 8.
8 turns into a 7 after that.
Now, you have to subtract 14 - 9 and 7 - 2. You're answer is 5.5.
You can check your work by doing inverse opperations:
By adding:
5.5 + 2.9 = 8.4
Or by subracting 8.4 - 5.5 = 2.9
Hope this helped you! If you need a better explaination or another example, I will be happy to help you! :)