is this a line we can add in the email template?

what exactly does "in Event model" mean ?

You can use the user account linked with the model you are working on and get the signature, for Example, in Event model you can get it in email template as below:

 
<t t-if="object.event_id.user_id.signature">
    <br />
    <t t-out="object.event_id.user_id.signature or ''">--<br/>Mitchell Admin</t>
</t>

odoo/mail_template_data.xml at 15.0 · odoo/odoo (github.com)