The meta query section in the Archive and Posts widget makes it possible to configure quite complex queries based on meta query keys and various value types.
To enable meta queries you need to choose a query type in the dropdown.
You can choose between the criteria being evaluated as AND or OR statements. If you choose AND all of the criteria must be true, if you choose OR only one criterion needs to be true. It is not possible to have nested criteria.
You can enter a meta key manually or use a dynamic tag depending on which plugins you have activated.
You can compare using the standard methods. If you choose between you can enter two values.
You can enter values manually or use various dynamic tags that is available. Not all values will work with the query so you need to try for your self. It depends on what the dynamic tag returns.
When you are using the Archive widget with the filter widgets you will in most cases be using the Query Parameter dynamic tag.