A terrarium is a box made of glass for animals and plants. The terrarium is something similar to an aquarium, but not for fish, but for other animals. Depending on what animals will live in it, the terrarium looks different. The word terrarium comes from the Latin word "terra", which means land or earth.
The terrarium is named after the landscape that is recreated. In a desert terrarium, for example, the animals should feel like they are in a desert. Such a terrarium is needed for animals that live in deserts in nature. In the terrarium there can also be places with water: This is then an aquaterrarium.
If you build a terrarium, you want to keep animals in the house. These are then special animals that can not simply live in the apartment. They would die or damage the apartment. Some animals are even dangerous to humans, like some types of snakes and spiders.
You also see terrariums in zoos and pet stores. Often people want to keep animals separate, so they don't put them in one big enclosure. They might eat each other. Some terrariums are also for quarantine: you separate the animal from others for a certain time. During this time you observe if the animal is sick.