Summon items are items that can give you a legendary egg. It can only be used once.
This item can be obtained through a scour mission and premium/ultimate membership. To use this item, you need to be at least on Senior rank or have 200 pokemon in your pokedex. You have to have a empty slot in your party in order to perform this action.
To use a summon item, you have to go to your farm page > tools. There, a new link will be unlocked. Click on the summon items and a list of your summons will be shown. You can then use it.
Remember, if you already own one legendary, you have to buy a wooden token from the shop. If you hatched it before you bought wooden token, the legendary pokemon will run away to the shelter.