what iOS is - its the operating system of the iPad (any iDevice, actually). It's what makes it run. So, I'm guessing you have an older, outdated iOS on your iPad, given you get those notices.
You can check what iOS version is on your iPad by going to the General section of your Settings app (the gear-shaped icon). Once there, on the right side, the very top line is titled "About." Press that line and somewhere in the next page is a line titled "Version." Thats the iOS you are on.
You can easily upgrade by connecting your iPad to your iTunes (make sure iTunes is current - get the lated version from Apple.com). However, you'll want to do a few prepatory steps first before the upgrade.
So, once the iPad is connected, you will see it show in the left column of iTunes. Right click on that line and, in the pop-up options, select "Transfer Purchases." This will make sure all the apps and data for them are updated in iTunes.
Then, make a back-up of your iPad by right-clicking again on the iPad line and selecting "Backup."
Once that is done, ensure you are on the main Summary page of iTunes. You should see a button titled either "Update" or "check for update." Press that button and iTunes will do the magic required to put the most current iOS version on your iPad.
At some point in the process, when prompted, you will select "restore from backup." This will enable iTunes to use that backup you just made to put your iPad back the way it was. Except - it'll have the newest iOS on it...
After all that is done, you can then go back to the App Store and get that app that requires iOS 5 or higher.
Good luck and keep us posted on how it goes.
Marilyn