Some liquids are more viscous than others. This means that they are thicker and flow less easily.
In terms of particles, viscosity is how easily the particles of the liquid move over each other. If the particles move over each other easily, the liquid is not viscous. But if it is harder for the particles to move over each other, the liquid is viscous.
Viscosity changes with temperature. A warm liquid will be runnier than the same liquid cold. That is because the particles have more energy in a warm liquid, and can move around more easily.
Viscosity can be used to predict how a liquid will behave in different pipes and bottles. We can then design containers for those liquids, based on those properties.
For example, consider ketchup. Ketchup used to be sold (and sometimes still is) in hard glass bottles. It was really hard to get ketchup out of the bottle because it is quite viscous. Instead of flowing down the side of the bottle, due to gravity, it would stick to the sides. People would try to shake the bottle, or whack the end of it, then would often get a giant blob of ketchup, far more than they wanted.
Nowadays, most ketchup is sold in squeezy bottles. This means that we are using pressure to get the ketchup out of the bottle; the pressure exerted is a stronger force than gravity, so can move the viscous liquid more effectively. There’s no need to use squeezy bottles for runny liquids like juice though, as they flow fine out of glass bottles.
Some scientists say that even solids flow, they are just very viscous. Flow has been observed in rocks, such as granite, plastic and glass. It’s just very very slow. Gases also have viscosity, they are just incredibly runny.