Suction is the creation of a partial vacuum, or region of low pressure. The pressure gradient between this region and the ambient pressure will propel matter toward the low pressure area. Physicists consider the notion of "suction" to be specious, since vacuums do not innately attract matter.
Higher pressure of surrounding air can push matter into a vacuum but a vacuum cannot attract matter. At zero air pressure, such as in space, suction would have no effect. However, most humans live at air pressure near 101.325 kPa (14.7 lbf/in² or 760 mmHg), which is the average atmospheric pressure at sea level. (It is lower at higher elevations.)
Suction in biology
Infants, and all baby mammals, are born with a sucking (or suckling) reflex, which they use in nursing liquid foods, such as milk. They do not have to learn this reflex, because it is instinctive. Some adult animals use suction in drinking, as do humans when using drinking straws. In breathing, the diaphragm muscle is used to expand the lungs, allowing air to enter due to the outside air pressure.
Trivia
Some physicists consider the notion of "suction" to be apocryphal, since vacuums do not innately attract matter. For this reason, a common joke among physicists is that, "There is no such thing as gravity: the earth sucks." (In fact, atmospheric pressure is set by an equilibrium between the Earth's gravity and the outward pressure generated by the concentration of air. If there were no gravity, the atmosphere's pressure would result in its outward dispersal into space.)
