When you sweeten something, you make it taste sweeter. To sweeten your lemonade, just add more sugar.
To literally sweeten something, like your muffin recipe or a cup of tea, all you need to do is add something that tastes sweet, like honey, molasses, sugar, or even apple juice. When you figuratively sweeten something, you make it more appealing or acceptable. The phrase "sweeten the pot" or "sweeten the deal" comes from poker, in which it means to add more bets (or money) to the game.