Well, you know how when you're really thirsty you like to drink water or juice or something else to quench that thirst, right? Well, a long drink is kind of like that, except it's usually made with alcohol mixed with other things like soda, juice or fruit flavors.
So let's say you want a long drink. The bartender will mix some vodka, gin, rum or other kind of alcohol with some soda or juice in a tall glass filled with ice. Then they might add a slice of lemon or lime to make it taste even better.
It's called a "long" drink because it's usually served in a tall glass, just like when you drink water or juice from a tall glass. See, you're already a little bartender in the making! But remember, since long drinks have alcohol in them, they are just for grown-ups, okay?