이

Marks the subject, after a consonant

Marks the word that is the subject — the thing doing the action or fitting the description. This is the shape used after a word ending in a consonant; 가 is the exact same marker after a vowel, and Korean speakers pick between them purely by sound. Against 은/는, which announces what the whole sentence is about, this one points at the specific thing that is true or new: it is what you hear when someone identifies, corrects, or reports something the listener did not already have in mind.