Depreciation rules require you to spread the costs of business assets (over a certain amount) over a number of years rather than fully deducting the costs in the year they are purchased.
It can be beneficial to expense some items upfront, and fortunately, the IRS offers some ways to write off full cost in one year. Depreciation can be more complicated than your average deduction, which is why we recommend reading: https://www.irs.gov/taxtopics/tc704
