这 + the polite measure word 位 (wèi) = 'this person' (with respect). Compare neutral 这个人 (zhège rén, 'this person') vs. polite 这位 — use 这位 when introducing or addressing someone you want to show respect to.
usage
Standard in introductions: 这位是… ('This is…'). Almost mandatory when introducing a teacher, elder, or guest. The plural is 这几位 (zhè jǐ wèi).
