Do you know what a secret is? It's something that you shouldn't tell anyone. Just like you have secrets, computers and phones also have secrets that they shouldn't tell anyone. But sometimes bad people try to find those secrets and use them to do bad things.
To keep those secrets safe, grown-ups called "cybersecurity experts" make sure that the computers and phones have strong locks so that bad people cannot get in. However, sometimes they find that bad people might sneak in through a secret door that nobody knew about and yell "hey, we need to fix this so bad people cannot sneak in!". When cybersecurity experts find that they quickly send a message to all the people who make computers and phones saying "Hey everyone, there is a secret door that bad people might sneak in, so make sure you fix it as soon as possible!".
That message is called "information assurance vulnerability alert," which is just a fancy way of saying "warning everyone to fix a secret door that bad people might use." This way, all the people who make computers and phones can fix the secret door so that no bad people can break in and steal the secret.