The short form of "they will." It is what people actually say — in speech, the full "they will" is rare and sounds slightly heavy or emphatic, so "they'll" is the normal, unmarked choice: They'll be here at six. Because "they" covers people, animals, and things, so does this: They'll arrive tomorrow (the guests); They'll break if you drop them (the glasses). It also does the modern singular use — one person whose gender you do not know or do not need to state: If anyone calls, they'll leave a message. In writing, it belongs to friendly, everyday registers — emails, messages, blog posts, dialogue. Formal writing (an academic paper, a legal notice, a business report) usually spells out "they will." One useful thing: the apostrophe stands exactly for the missing letters (wi-), and the sound collapses too — it is one syllable, roughly "THAYL," not two.