It is also hard for them to concentrate or remember things sometimes. The really bad tiredness happens because people with CFS can't sleep very well. Your body needs very deep sleep every night so you can do a lot of stuff during the day. It's kind of like a toy with rechargeable batteries. If the toy battery doesn't get charged enough, the toy will not work very well.
Good sleep is what recharges people batteries.
There are some medicines the doctor can give your mom to help the symptoms not feel so bad, but they don't make the disease go away. People with CFS have to rest a lot and try not to do too much working or playing. There are times when your mom feels really, really bad and other times when she feels kind of okay. Sometimes she gets very sad or angry because she can't do all of the things she wants to. That way your mom have more energy to do something fun with you.