HI @itsPhil1
it should be:
{{ product.metafields.custom.nameofmetafield.value }}
Along with the curly brackets the key nameofmetafield needs to be the actual key of the metafield you created and not a generic placholder used for examples sake.
This is IF the metafield is on the product itself, and the “product” is the context of the variable in the collection template or product grid.
meaning it could be current_product or card_product , etc etc instead of just product but you need to read the theme code to be sure