To add default filter to grid in Magento2?

If you define grid collection thought layout than you can use updater to add default filter.

<argument name="dataSource" xsi:type="object">
    Tutorial\SimpleNews\Model\Resource\News\Collection
    <updater>Tutorial\SimpleNews\Model\Resource\News\Collection\Updater</updater>
</argument>

and

<?php
namespace Tutorial\SimpleNews\Model\Resource\News\Collection;

class CollectionUpdater implements \Magento\Framework\View\Layout\Argument\UpdaterInterface
{

    /**
     * Update grid collection according to chosen order
     *
     * @param \Tutorial\SimpleNews\Model\Resource\News\Collection $argument
     * @return \Tutorial\SimpleNews\Model\Resource\News\Collection
     */
    public function update($argument)
    {
        $argument->addFieldToFilter('you_field', 'value');

        return $argument;
    }
}

or Extend Grid block

class Grid extends \Magento\Backend\Block\Widget\Grid
{
    protected function _prepareCollection()
    {
        if ($this->getCollection()) {
            foreach ($this->getDefaultFilter() as $field => $value) {
                $this->getCollection()->addFieldToFilter($field, $value);
            }
        }
        return parent::_prepareCollection();
    }   
}

you need to add this inside the arguments tag:

<argument name="default_filter" xsi:type="array">
    <item name="field_name_here" xsi:type="string">value here</item>
</argument>

if your arguments are contained in this block

<block class="Magento\Backend\Block\Widget\Grid" name="some.name.here" as="grid">

you need to create your own class that extends Magento\Backend\Block\Widget\Grid like this:

<?php 
namespace Namespace\Module\Block\Adminhtml\Whatever;

class Grid extends \Magento\Backend\Block\Widget\Grid
{
    public function _construct()
    {
        parent::_construct(); 
        if ($this->hasData('default_filter')) {
            $this->setDefaultFilter($this->getData('default_filter'));
        }
    }
}

and modify the block tag above to

 <block class="Namespace\Module\Block\Adminhtml\Whatever\Grid" name="some.name.here" as="grid">

if you already use a custom grid and not the default Magento\Backend\Block\Widget\Grid you don't need to create the class.
You just need to copy the _construct method from above in your class.