What is a linguistic marker?

A linguistic marker is a meaningful sound (morpheme) indicating the grammatical function of of a Word, phrase or sentence. Some languages use markers more than others. For instance, Russian uses more markers than the current use of English which has fewer markers than it did in the past. The unmarked form of the Word is what you usually find in the dictionary.

Word: The sound "ed" in the Word "worked" marks the verb as being a past tense.

Phrase: The sound "on" in the phrase "on the table" is used to indicate location ("on" answers the question "where").

Sentence: Interrogative sentences are usually marked by the intonation. When speaking the sentence, "Are you asleep?", the speaker will mark this as as a question by a rising intonation.