A bowlful is the amount that fills a bowl. When you pour yourself a bowlful of cereal in the morning, you're filling the bowl up to a reasonable level. If your mom serves you a bowlful of soup for lunch, she's giving you roughly one bowl's worth.

The word is useful because different bowls hold different amounts. A small rice bowl might hold just a cup of food, while a large mixing bowl could hold several quarts. So when a recipe calls for “a bowlful of berries,” it matters which bowl you use. A bowlful from a tiny dessert bowl won't be the same as a bowlful from a giant salad bowl.

You'll also hear people use bowlful to describe generous portions or abundance. If someone talks about eating “bowlfuls of ice cream,” they're emphasizing that they ate a lot. Notice that the plural is bowlfuls, not “bowlsful,” even though you're talking about multiple bowls.