What AQI level is it acceptable to walk around without mask in?
California Ocupational Safety and Health Standards require that employers provide their workers with proper respirators if the AQI PM2.5 (air quality related to fine particles) is 151 or greater, which is the unhealthy level of concern. AirNow lists N-95 masks or P-100 respirators as options for going outdoors for relatively short periods of time in these cases.

Below that is AQI 101-150, which is unhealthy for sensitive groups. If you are not a member of a sensitive group, you'll likely be fine, and don't need to worry as much about walking around with a mask. Below that at AQI 51-100 is moderate, which most people should be fine with, excluding unusually sensitive people. AQI 0-50 is good, and poses little or no risk to anyone.

In cases of exposure, you may feel eye, nose, or throat irritation; coughing; phlem; chest tightness; or shortness of breath. These should go away when in an area with better air quality. airnow.gov recommends staying in a house with filtered air if possible. Otherwise, make sure your air conditioner's fresh air intake is closed and the AC filter is clean. Or, use a personal air filter in the room you're staying in, but don't use an air filter that generates ozone.

Note that AQI only measures ozone, particle pollution, carbon monoxide, and sulfur dioxide. It won't be a useful measure if you're worried about other chemicals or substances in the air.
How is sensitive group defined in the context of AQI?
In the context of the Air Quality Index (AQI), sensitive groups refer to individuals who are more vulnerable to the health effects of air pollution. These groups include:

Children: Because their lungs are still developing, children are more susceptible to the harmful effects of air pollution. Exposure to air pollution can cause asthma, reduced lung function, and other respiratory problems in children.

Elderly people: As people age, their immune system weakens, making them more susceptible to the health effects of air pollution. Elderly people with existing respiratory or cardiovascular diseases are at higher risk of experiencing negative health effects due to air pollution.

People with pre-existing respiratory or cardiovascular conditions: People with conditions such as asthma, chronic obstructive pulmonary disease (COPD), or heart disease are more vulnerable to the health effects of air pollution. Exposure to air pollution can worsen these conditions and lead to hospitalization or even death.

Outdoor workers: People who work outdoors, such as construction workers or landscapers, are at increased risk of exposure to air pollution. Prolonged exposure to air pollution can cause respiratory problems and other health issues for outdoor workers.

Pregnant women: Exposure to air pollution during pregnancy can harm the developing fetus and increase the risk of low birth weight, premature birth, and other adverse birth outcomes.

In summary, sensitive groups are individuals who are more vulnerable to the health effects of air pollution due to their age, pre-existing health conditions, or occupation. It is important to protect these groups from exposure to air pollution by taking measures to improve air quality.