Some plants will interact well with one another and increase in their rate of growth whereas other plants may not get along well together and growth will be delayed.
Companion planting is the process of planting different types of plants together so that they will help each other to produce better crops. Each plant will have its own characteristics in attracting or repelling certain insects or having certain chemical substances in their roots.
If the relationship between the 2 types of plants is beneficial, then the growth of the plants will be faster and it will result in better crops. However if the relationship between the plants is not beneficial, the plants may grow more slowly or may not survive at all.
Companion planting can help reduce the use of chemical pesticides to protect the plants. The odor given out by certain plants can help to repel some harmful insects whereas some other plants are able to attract beneficial insects that can help with pollination, amongst other things. As an example rose plants are typically damaged/destroyed by aphids (the greenfly), but the smell from the marigold plant deters aphids and instead attracts hoverflies which kill the aphids.