Celery leaf comes from, as you might have deduced, dried leaves of the crunchy celery plant. Grassy and hay-like, even a little bitter, the flavor of celery is reminiscent of fennel and anise, but much, much milder.
Dehydrated celery leaf is twice as strong as the fresh vegetable, and lends strong and concentrated celery flavor to your cooking. Use only half as much dried celery leaf as you would fresh, i.e. 1 Tbsp = 2 Tbsp fresh!
Use in soups, potato salad, brisket, coleslaw and anywhere a sharp celery flavor is needed. In foods that simmer over time, like soups, celery leaf is an innovative addition. Like many other bitter greens, the celery leaves sweeten with longer cooking times. Celery leaf brings a bright new taste to mayonnaise-based salad dressing.
Great to have on hand when you need just a bit of celery flavor to brighten a recipe. Also useful when cooking for toddlers or, to put it nicely, for "selective" eaters who are averse to the texture [and occasional stringiness] of fresh celery. Celery leaf makes a great substitute, also, for cilantro in Mexican recipes.