Two and a half years on, Bootstrap 5 (still in beta as of 7 January 2021) now looks to have a better solution in the ability to nest tables.
https://getbootstrap.com/docs/5.0/content/tables/#nesting
It looks like a subtle but important expansion on the answer provided by bitstarr, particularly because it will allow you to choose whether you want to inherit styling of the parent table or not.
Nesting is just achieved by making sure that you include a "colspan" value in a row, then add another table inside that row.
As a result, the first two rows or your intended outcome (the heading and the first row of content) would look something like this:
<table class="table table-striped">
<thead>
<tr>
<th>head</th>
<th>title</th>
<th>title</th>
<th>title</th>
<th></th>
</tr>
</thead>
<tbody>
<tr>
<td>
<input type="checkbox">
</td>
<td>content</td>
<td>content</td>
<td>content</td>
<td rowspan="2">white</td>
</tr>
<tr>
<td colspan="4">
<table class="table mb-0">
<tr>
<td>
Some other text that can be as long as a row
</td>
</tr>
</table>
</td>
</tr>
</tbody>
</table>