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.
Have alot of seedlings or rooted cuttings, but not enough pots? Easy, you can make your own, plant them with your rootlings, and they break down. You can buy moulds or make your own using bottles, tins or any cylindrical object that fits the size you need. Follow the directions, you may need some practise, save money and reduce your plastic use.
Choose moulds of different sizes in order to make a range of pots suitable for all your seed sowing and plant raising activities.
Cut strips of newspaper wide enough to fold halfway across the mould base, and roll the paper round to make 4-6 layers per pot.
Wrap tightly to create firm pots, then start folding the paper over the base so there is no gap for any compost to fall through.
Wooden kits have a crimping block that helps create a crease to hold paper rigid, or just push the base firmly onto a flat surface.
Carefully slide the paper pot from the mould. Stand pots in a seed tray, ready for sowing.
Rest the pots in trays of damp compost. This ensures the paper always remains moist, allowing roots to grow through the sides. Make sure the pots are moist before planting, this aids the breaking down process.