It means you are not fully exchanging the air in your lungs. You are not completely exhaling before inhaling. It's big problem with smokers as with people with emphysema. I have that problem sometimes, especially when not paying attention to my breathing.
Do pursued lipped breathing a few times. [See Resource section, above.] I also do it for a breathing exercise twice daily. Make sure you are exhaling twice as long as you inhale. Blow up a balloon a couple times will also help. [But I have troubles with that one.]
Hope that helps some. Others may have a better explanation than that.

@A MyCOPDTeam Member
Air trapping is a common feature of emphysema and occurs when, during exhalation, the lungs don’t empty normally. More air is retained in the lungs than should be. With each subsequent full inhalation, more and more air can remain in the lungs, leading to hyperinflation of the lungs and causing shortness of breath.
Hyperinflation becomes static or during exercice, this is the reason when walking we have to stop for some seconds/minutes to recover a "normal breath in and out" and then be able to walk again.
Air trapping creates bubbles/holes in the lungs, and as the emphysema progress, more and more bubbles where in air is trapped, and consequently hyperinflation, and severity of short of breath.
Solution: quit smoking (not even one cig) and take bronchodiltators meds which helps.
Hugs 😘
(ps. this is exactly what I'm suffering of).

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A MyCOPDT...

https://youtu.be/kdGaHN8Izpc?si=sb56NvbuWjxtv16U
Hello Julie. The explanation is simple IF you understand the Breathing process from a standpoint of AIR Pressures. The reason Pursed Lip Breathing helps is because it creates a BACK-pressure in the lungs. This helps "pop" the Alveoli cells to release more of the OLD gases during Exhale. As a reminder, the exhale should be forceful but intentionally SLOW. Use of a balloon may help too. You can do this Any TIME, any Place and IT will help you be prepared for a SOB episode.
be Well

ps. Pardon the commercial in this video. It's worth waiting for the INFO tho. 😉