Step 1). Create the Vendor/Module/etc/frontend/event.xml in your module

<event name="controller_action_postdispatch">
   <observer name="change_title_filter" instance="VendorModuleObserverFiltertitle" />
</event>

Step 2). Create the Vendor/Module/Observer/Filtertitle.php in your module

<?php

namespace VendorModuleObserver;

use MagentoFrameworkEventObserverInterface;
use MagentoFrameworkEventObserver;
use MagentoFrameworkViewLayoutInterface;

class Filtertitle implements ObserverInterface
{
    protected $context;
    public function __construct(
        MagentoFrameworkViewElementTemplate $context,
        MagentoFrameworkViewResultPage $resultPage,
        MagentoFrameworkViewPageConfig $pageConfig
    ) {
        $this->pageConfig = $pageConfig;
        $this->resultPageFactory = $resultPage;
        $this->context = $context;
    }  
    public function execute(MagentoFrameworkEventObserver $observer)
    {
        $layout = $this->context->getLayout();
        $pageMainTitle = $layout->getBlock('page.main.title');
        if ($pageMainTitle) {
            $blockfilter = $layout->getBlock('catalog.navigation.state');
            if($blockfilter){
                $selectedFilters = $blockfilter->getActiveFilters();
                if (!empty($selectedFilters)){
                    $filters = array();
                    foreach($selectedFilters as $filter){
                        $filters[] = $blockfilter->stripTags($filter->getLabel());
                    }
                    if(count($filters) > 0){
                        $activefilters = __("Your selection: ").implode(" - ", $filters);
                        $pageMainTitle->setPageTitle($activefilters);
                    }
                }
            }
        }
    }
}