If you deposit money in a traditional IRA before April 15, 2018, you can count it as a contribution either for 2017 or 2018. Any amounts deposited after April 15 only count as 2018 contributions.
If you make contributions that are counted toward 2017, and if you are otherwise eligible to make deductible contributions, then you take a tax deduction for the amount and it will decrease your tax owed or increase your refund.
It will also lock up your funds until you retire, so it's not exactly a free transaction. If you are saving money anyway, it may make sense to put it in an IRA, but if you might need the money in 5 years for a car or a house, you need to realize that putting into an IRA mostly makes it untouchable until you retire.
