To can or freeze tomatoes, or to make sauce or soup it is best to remove the tomato skins (and seeds) before doing so. Tomato skins and seeds are harder to digest and they do not cook down like the flesh does and will appear as seeds and strips of skin in your finished product. Blanching loosens the skin so that it can be easily removed.
Boil water in a pot deep enough to cover the tomatoes. Wash your tomatoes and remove any leaves. We like to core tomatoes and scoop out most of the seeds before blanching. Some folks blanch then core. Either way is fine but you’ll have less to do with a hot tomato if you do it first. Cut an X in the bottom end of the fruit, opposite the stem. Carefully place the tomato in the gently boiling water so you don’t splash yourself. We like to use tongs or large slotted spoon to lower the tomatoes into the water. Leave the tomato in the water for 30 seconds to one minute. When the skin starts cracking or rippling, lift the tomato out and place in a bowl of ice water. Let the tomato chill in the ice bath for a minute or two and the skin should slip right off. Be careful because the tomato will be very hot.