Use of ifconfig tag in layout xml?

The use of ifconfig is for condition which is coming from system config values. The value of this will be 0 and 1(boolean). According to that value block will be rendered. e.g.

<block ifconfig="custom_quickview/general/enable" class="Magento\Catalog\Block\Product\View" after="product.info.addtocart" name="goto.product" as="goto.product" template="Custom_Quickview::gotoproduct.phtml"/>

In this case if under system config custom_quickview/general/enable == 1 then only this block will be rendered else not.

The ifconfig attribute allows you to specify a path to Magento's configuration (e.g. dev/log/active) that will be evaluated as a boolean value (using Mage::getStoreConfigFlag()). If this configuration option is set to true the action method will be called, the block will be created or the content of the reference will be evaluated.

Basically it's a condition in that block to render it or not, this condition you will set it in Admin->Stores->Configuration->All that config boolean fields and it comes in this format:

ifconfig="section/group/field"