Onset is how long it takes to start feeling the effects of cannabis after you use it. It depends mostly on how you consume it.
Smoking and vaping: effects usually begin within a few minutes, since cannabinoids pass through the lungs into the bloodstream quickly. Effects often peak within half an hour and fade over a few hours.
Edibles: effects usually begin in 30 minutes to 2 hours, and sometimes later. The cannabis passes through your digestive system and liver first. Effects can last 6 hours or more and may feel different from smoking.
Tinctures: placed under the tongue, some people notice effects sooner than edibles; swallowed, they behave more like edibles.
Topicals: generally non-intoxicating.
Onset is also affected by whether you have eaten, your metabolism, your body and your tolerance. The same edible can feel different on different days.
The biggest risk with slow onset is "double-dosing": taking more because you do not feel anything yet, then having both doses hit together. Wait at least two hours after an edible before deciding to take more. Plan ahead, do not drive, and give yourself plenty of time.