According the kinetic molecular theory, gas in a closed container exerts pressure on the walls of that container because its molecules are in constant motion, which causes some molecules to impact the walls. The more the gas molecules impact the walls of the container, the greater the pressure.
The amount of pressure a gas exerts on a closed container is related both to the mass of the gas in the container and the temperature of that gas. The mass of gas matters because that determines how many molecules are in the container. The more molecules there are, the more often molecules will strike the walls. Temperature causes the gas molecules to move more quickly, which also increases the rate. A decrease in the volume of the container also increases the pressure of the gas.
Pressure is the amount of force one substance exerts on another substance for each unit of area. If it is within an atmosphere, the air outside of a container constantly exerts pressure as well. A balloon will only inflate if the pressure of the gas put into it is greater than the atmospheric pressure around it. On Earth, the average air pressure at sea level is 14.7 pounds per square inch.