Tolerance is the way your body adapts to cannabis with regular use. Over time, the same amount may feel weaker, and some people find they need more to get the same effect. This happens partly because the brain's CB1 receptors become less responsive with frequent THC exposure.
Tolerance builds at different rates for different people, depending on how often they use cannabis, how much and which products. Frequent use of high-potency concentrates tends to build tolerance faster than occasional use of flower.
Tolerance also matters for safety. Someone with a high tolerance may comfortably use an amount that would overwhelm a new user. That is why you should not base your dose on what a friend uses, and why people returning after a long break should start low again, since tolerance drops when you stop.
Many regular users take a "tolerance break," pausing for a few days or weeks to let their sensitivity return. Others use lower-potency products or smaller amounts.
Tolerance to some effects can differ from others. Even with high tolerance, cannabis can still impair driving and coordination. Never drive after use.