Phenological mismatch is when different living things, like plants and animals, don't do things at the same time as they normally do.
For example, let's say that bees typically gather nectar from flowers during a certain time of year. But, because of things like global warming, flowers might start blooming earlier or later than usual. If the bees are used to gathering nectar at a certain time, but the flowers aren't blooming yet or have already bloomed and are gone, then the bees won't have enough nectar to eat.
This can be bad for both the bees and the flowers. The bees might not have enough to eat, which can make them sick and might even cause them to die. The flowers might not be able to reproduce if they don't get pollinated by the bees.
Phenological mismatch can happen between lots of different living things, like birds that migrate at a certain time but find that their food isn't available yet, or even between certain types of insects and the plants they depend on. It can be really important to know about phenological mismatch so that we can try to help different living things stay on the same schedule and keep everything in balance.