I made a salad with only herbs.

This joke hinges entirely on a classic bit of wordplay, specifically a pun. The humor comes from the phonetic similarity between "thyme," a common herb, and "time," as in the concept of duration. The setup leads you to think about ingredients in a salad, making the herb context relevant, before the punchline delivers the unexpected twist.

Thyme, a fragrant herb often used in cooking for its earthy and slightly floral notes, is a staple in many kitchens. Meanwhile, the idiom "too much time on my hands" is a common phrase meaning one has an excess of free time, often leading to idleness or engaging in trivial activities. The unexpected connection between a culinary ingredient and a state of leisure is what gives the joke its lighthearted absurdity.

The comedic effect is achieved by cleverly substituting the herb for the word in the well-known idiom. The image of someone making a salad composed solely of herbs, particularly one as strongly flavored as thyme, is already a bit silly, setting the stage for the equally silly and unexpected pun that ties it all together with a knowing wink.