For IRS purposes, disabled means "unable to perform gainful work". Sometimes, people are medically disabled or unable to perform their original job, but would be able to perform other "gainful" work, and therefore are not disabled for IRS purposes.
On your tax return, checking the box will enable a small additional deduction. It would also make someone else eligible to claim a dependent care credit for you, if you are unable to care for yourself. There aren't many other deductions.
Your income may not be taxable in which case you don't have to file, or you could prepare a return with your information and file it just to get confirmation that you owe no tax. Turbotax should be free if you don't have additional income (like investments or IRA withdrawals).
If you have other income besides a disability check, it may be taxable even if your disability check is not taxable.
