Being "vulnerable" means being weak or easy to attack.
You can use "vulnerable" to talk about an army's situation in a war:
The Empire waited to strike until the Rebellion was vulnerable.
You can also use it to talk about diseases:
When you don't get proper sleep, nutrition, and exercise, you're more vulnerable to infectious diseases.
A person can be emotionally vulnerable as well. That means that they are easily upset or saddened:
She's really vulnerable right now after their breakup.