Try using this kind of approach. 

t t-if="record.state.raw_value == 'offer_r'">

​div class="o_kanban_field">

​ ​Best Offer: 

​ ​field name="best_offer">

​/div>

/t>

(I removed the tags because submitting the answer would cause the code to disappear)

Here is the exercise given in 15th chapter. First of all ill explain the o_kanban_field. I used it to separate the fields because they were in the same line and you can write your label straight in the div(If you want to make the text bold surround the text with tag). The t-if is for the best offer to be visible only if the state is offer received.

Hi,

To add static description strings next to the actual model values in a Kanban view in Odoo, you can achieve this by using the t-field attribute. The t-field attribute is used for defining the string to display in the field on the Kanban card. Here's how you can do it:

Add a t-field attribute to the field tag, and set it to a string that includes the description and a placeholder for the actual field value. For example:

<field name="expected_price" t-field="('Expected price: %s' % record.expected_price)"/>

In this example, the string "Expected price: " is displayed along with the actual value of the expected_price field.

<kanban>
    <field name="name"/>
    <field name="expected_price" t-field="('Expected price: %s' % record.expected_price)"/>
    <field name="best_offer" t-field="('Best offer: %s' % record.best_offer)"/>
    <!-- Add more fields with descriptions as needed -->
</kanban>

Hope it helps