Errors and omissions excepted means that if there are mistakes or things left out, the person or company who wrote or said something is not responsible for them.
Imagine you have a toy box with lots of different toys inside. You tell your friend that you have a red car in your toy box, but you forget to mention that it's missing a wheel. Later, your friend comes over and looks in your toy box. They see that your red car is missing a wheel and they get upset because you didn't tell them.
But then, you remember that you said "errors and omissions excepted" when you told your friend about the red car. This means that you're not responsible for forgetting to mention that the car was broken. Your friend might still be upset, but they can't blame you for leaving out that important detail.
So basically, when someone says "errors and omissions excepted," it's like them saying "I did my best to give you accurate information, but if there are any mistakes or things I left out, it's not my fault."