All gardening advise about growing tomaotes will say to cut side shoots from Tomato plants - but why and what are side shoots? Why does this matter when  growing tomatoes?

Why, it is necessary because Tomatoes are very vigorous plants and you need to contain the plant to the main frame work. If every side shoot was allowed to grow the plant would be massive, all leaf and few flowers with a resulting poor crop of tomatoes.  When growing tomatoes not only should you cut off all side shoots, but also thin and reduce the amount of foliage particularly as the fruits form.

 What are side shoots?  See below the scissors point to the side shoot.

For more about growing Tomatoes