To find a bird's wingspan, you need to measure the distance from the tip of one outstretched wing to the other. An eagle has a much larger wingspan than a sparrow.
Anything with two wings has a wingspan, which is one way to get a sense of its size. So birds and butterflies can be measured for wingspan, but so can airplanes. A flying squirrel can have a wingspan of over a foot, and some hummingbirds have wingspans that are barely over an inch. Sports commentators often mention the wingspan of basketball players — in this case, they're referring to the distance between a player's outstretched fingertips.