This only applies to the upright type of tomato and not the bushy types.
With Cordon tomatoes, the upright varieties as the plant grows, it is essential to remove all new side shoots, which appear at the junction between the upright stem and the branch. Look at the plant's main stem, and where the branches grow, any little shoot appearing at the junction of the upright stem, at right angles with the branch needs to be removed.
This is because tomatoes are very vigorous and they will not only grow up and up, if allowed, they will also grow many side shoots and become a tangled mass of leaves and fruit less.
In the image left, the arrow shows the side shoot which grows at the junction. During the growing season, the plant will continually produce side shoots at the junction of the plant and stems; keep removing them.
By doing this, you contain the plant's growth; you do not want a lot of extra branches growing out sideways. Unless these shoots are removed, the plant because it is so vigorous, will produce a mass of leaves and shoots. By restricting growth, it will produce more fruits. Below is a short 2mins 30 sec video showing how to remove the side shoots.
The next step in tomato growing is to contain the plants' upward growth.
Stopping off Tomato plants