This is a huge field, put simply it means you grow plants together to achieve a desired result.You may want your plants to grow more strongly, a classic example of this is growing tomatoes and basil together. To deter harmful insects you can plant tagetes-African marigolds.
In some countries where resources like water are few, people plant different types of vegies together to support each other. Corn and beans is a good example, the beans climb the corn, return nitrogen to the soil, both benefit. I've personally done this, not only did it look good, the results were very positive.
Over time we can explore more ideas for planting. Meanwhile think about th classic, roses and garlic, the roses benefit, and you get yo eat the garlic.
Remember companion plants act as deterrents, they don't kill kill harmful predators.