Of the highest quality — the best there is. It comes from the picture of a chest of drawers, where the top drawer is where you keep your finest things. Used before a noun it means excellent: top-drawer talent, a top-drawer performance. People also say it after the verb, usually without the hyphen — "her cooking is top drawer" — and that use is common in British speech, where it sounds a bit like praise from an older, well-spoken person. There is an older social meaning too, mostly British and now dated: "out of the top drawer" meant born into the upper class. That one carries a whiff of snobbery, so recognise it rather than say it. The word is informal and warm; it is praise, not a technical rating.