This rule is a boon for small landlords.
As I understand it (I am not a professional; do your own research), when
1) The individual expense is less than $500
and 2) Your total repair expenses for the year are less than $10,000
you can deduct the expense as a repair rather than depreciating it as a capital expense.
You will need to write a "company policy" that expenses of less than $500 are to be treated as repairs rather than capital expenses. You will also need to include a "de minimis" election with your tax return.
The idea is to save the time and trouble of figuring out whether an expense is repair/maintenance or capital in cases where the overall effect on taxes owed is small.
Don't abuse it.
Companies that are large enough to have their finances done by an accounting firm have had a safe harbor like this for several years. Theirs works for larger dollar amounts.