Marked nominative is a special way of talking about the subject of a sentence in some languages. It's like wearing a special hat that says "I'm the subject". It helps us understand who's doing the action in the sentence.
For example, let's use the sentence "The cat is playing with the ball". In English, we don't use a special hat to show the subject, but we know that the cat is doing the action of playing with the ball.
In some other languages, like Russian or Hindi, they use a special hat (or case) that shows who the subject is. It's called "nominative" and when it's marked, it means it's wearing a special hat that makes it stand out. So, in Russian, the same sentence about the cat and the ball would be "Кот играет с мячом" (kat igrayet s myachom), where "кот" (kat) is wearing the special nominative case hat, showing us that it's the subject doing the playing.
It might seem complicated, but it helps make sure everyone knows who's doing what in the sentence, even when the sentence gets longer and more complex.