Salty. Sweet. Sour. Bitter. Umami.
These are the five basic tastes. We're all pretty familiar with the first four, but what is umami? In 1907 according to this site, "Professor Kikunae Ikeda of Tokyo Imperial University was thinking about the taste of food: 'There is a taste which is common to asparagus, tomatoes, cheese and meat but which is not one of the four well-known tastes of sweet, sour, bitter and salty.'"
We might describe the flavor as "savory" or "earthy". Ikeda eventually invented a seasoning which carried umami flavor in a form which could be used as a seasoning. Glutamic acid (or glutamate) is the base form of the umami flavor, which Ikeda transformed into monosodium glutamate or MSG. Unfortunately, MSG also causes adverse reactions in a large group of people. When you get a headache after a bowl of chow mein from your local Chinese take out, you can blame Kikunae Ikeda.
But don't give up on umami. There are other ways to add this flavor to your foods and create an unexpected richness.
Add some of one of these foods to a sauce or a stirfry or saute to add that difficult to describe something that makes the dish - Thai fish sauce, shiitake mushrooms, Worcestershire sauce, truffles (and truffle infuesd oils and butters), parmesan, oily fish such as anchovies, mackerel, blue fish, or sardines, Japanese bonito flakes, and the food Ikeda used to isolate glutame - kombu broth (kombu is a type of seaweed). All have a strong umami component.
Properly aged beef develops an umami flavor that fresher beef doesn't have. If you can't get aged beef for your dishes, you might consider adding a little of one of the ingredients above to the dish to enhance your fresh beef's flavor.
Substitutions are best when they're unexpected. Image a dissolved anchovy or a dash of fish sauce in a tomato based pasta sauce or a dash of truffle viniagrette on creamy leeks (a dish we had and loved at Campagne in Seattle).
If you don't add umami flavors to your food, you can always drink them. Certain wines also have umami flavor in them, primarily rich broad reds like Australian Shirazes. Some Chardonnays and Champagnes can have the flavor as well.
Picture from Australian Broadcasting Company site