A dollop is a small, indefinite amount of something. You might ask for a dollop of sour cream on your taco.
English has many words for amounts of something that aren't exact. For example, a load is a lot of something, but a dollop is a little. If you're watching your weight, you may only want a dollop of dressing on your salad. Most people use a dollop of shampoo when washing their hair. You can't measure a dollop, but it's should be a fairly small amount. If you use a whole bottle of ketchup, that's far more than a dollop.