There are some specific variables accessible to make template reports.
1. docs : records for the current report
2. doc_ids : list of ids for the docs records
3. doc_model : model for the docs records
4. time : a reference to time from the Python standard library
5. user : res.user record for the user printing the report
6. res_company : record for the current user's company
For example see below code

  <template id="report_invoice">
        <t t-call="report.html_container">
            <t t-foreach="docs" t-as="o">
                <t t-call="report.external_layout">
                    <div class="page">
                        <h2>Report title</h2>
                        This object's name is <span t-field="o.name"></span>
                    </div>
                </t>
            </t>
        </t>

</template>

Note- If you wish to access other records/models in the template, you will need a custom report.