Many plants benefit from being potted on as they are growing and none more so than tomatoes. Usually they need to be potted on about 2/3 times before planting intheir final pots. Potting on just means repotting into a slightly larger pot. The photo advise shows the various stages. First is the plant ready to pot on; this one in the photo is well overdue the plant is much too large for the pot. What size pot, don't be tempted to take into too larger pot the plant will grow best if its just upgraded so of the pots shown, the middle sized on is better. Lift gently out of the pot, easy into the new pot and firm down. Make sure there are no air pockets and the plant is firmed in. Plants can only grow in soil, if its not firmed down and there are air pockets the roots will struggle. As always water after transplanting .