Baldur's Gate was a great game, and many wished it would never end. Tales of the Sword Coast was a small expansion to extend game playing time a bit.

You need the full version of Baldur's Gate in order to be able to play this expansion. In essence it adds more to the end of the game - letting your character gain another few levels, and go on more quests.

You get some new locations to explore, plus new quests in existing locations. There are new, interesting spells, armor, weapons and more. If you've already maxed your character out in the base Baldur's Gate, it lets you have some more fun with the same character, extending your gameplay fun.

On the down side, you can only increase by maybe one or two levels, and the additional gameplay is still pretty short. It's definitely not that you're getting a new, full game. You're just extending existing gameplay a bit. As long as you realize that, you'll enjoy it quite a bit!

Baldur's Gate Review
Baldur's Gate Walkthrough