A well-stocked place has plenty of things in it, and a good variety of them. It is used most about places that hold supplies: a well-stocked fridge, a well-stocked kitchen, a well-stocked bar, a well-stocked shop, a well-stocked library, a well-stocked first-aid kit, a well-stocked pond (with fish put in it).

Two ideas live in the word at once, and students usually catch only one. There is amount — a lot of it, you will not run out. And there is range — not just a lot of one thing, but a sensible spread of what you might need. Two hundred bottles of the same beer is not a well-stocked bar. Twenty different bottles is. If you only mean amount, 'full' is the word you want.

There is also a quiet feeling of readiness in it. A well-stocked cupboard is one where you could cook dinner tonight without going out; a well-stocked shop is one where you can trust that what you came for will be on the shelf. That is why it turns up so often in travel writing and rental listings — the flat comes with a well-stocked kitchen — and in advice about typhoons or storms: keep a well-stocked pantry.

Ordinary, friendly English, common in speech and writing alike, and always positive.