A placeholder image is a default image for a store to draw attention to the need for an actual image. Its temporary image for an actual image.
Sometimes product actual images are not available at that time placeholder images to represent the product in a site for each place like catalog product page, listing page, mini cart and cart page.
Placeholder is a temporary image for a missing image of products. By default Placeholder image in Magento 2 is Magento logo and they will be stored under Magento Catalog module.
You can refer link for, Get Placeholder image url programmatically in Magento 2.
You can see images for placehoder under the vendor/magento/module-catalog/view/base/web/images/product/placeholder folder.
Native Magento 2 has four types of placeholder images,
How to upload the placeholder image via Magento Admin panel?
Step 1: Login to your Magento 2 Adminpanel,
On the Admin Panel left sidebar,
Stores > Settings > Configuration.
In the panel, under Catalog, choose Catalog.
Step 2: Upload placeholder images
Expand the Product Image Placeholders section. Click on Choose File to upload the custom placeholder image for a site. You can use the same or the different images for specific image role.
Step 3: Save the configuration
Click on Save to complete.
Clear the cache from Cache Management.