An IRA is your own money. You may withdraw any amount at any time for any reason. If this is a traditional (pre-tax) IRA, the money will be taxed as ordinary income, and if you are under age 59-1/2, there is a 10% penalty for early withdrawal unless you meet an exception. But you can withdraw any time you want.
Once you reach a certain age (73 in your case), you are required to withdraw at least something each year. This is called a Required Minimum Distribution (RMD) and the reason is that you are not supposed to leave all this tax-free money for your heirs, you are supposed to spend it during your own retirement. RMD is based on statistical life expectancy. So if your statistical life expectancy when you turn 73 is 20 more years, you must withdraw at least 1/20 of your IRA that year, then 1/19th in the year you turn 74, and so on. However, you can always withdraw more if you want to, and you can withdraw before your RMD beginning age if you want to.
