Software assurance is like making sure your toy is good and safe to play with. Just like when you ask your mom or dad to check your toy, a company or organization also needs to check their software to make sure it's good and safe to use. They do this by testing the software over and over again to make sure it works correctly and doesn't have any bad bugs or errors.
Sometimes the software might have weaknesses that bad guys or hackers can use to do bad things. So, software assurance also includes finding and fixing these weaknesses, so nobody can use them to do bad things.
Overall, software assurance is like checking your toy to make sure it works well and is safe to play with, just like a company checks their software to make sure it works well and is safe to use.