Personalization tokens allow you to personalize your content to your user based on their contact/company information from their record in HubSpot. For example, if you use the First Name personalization token, that token will pull the information from the first name contact property within their HubSpot contact record.
In order for personalization tokens to work properly, the contact must have a contact record within your HubSpot Sales or Marketing account. Personalization tokens will not be able to pull information from a contact that lives outside of HubSpot, like in Salesforce or Gmail.
In Sales email temples, you must use personalization tokens by clicking on the Contact token or Company token dropdown menus in the template composer window. If you manually insert your own tokens using curly brackets, those won't function. Instructions for using personalization tokens in sales templates can be found here.