A Bloom Summon (Burūmu Shoukan) is the method of Summoning a Bloom Monster. To Bloom Summon a Bloom Monster, you must control a face-up Seedling Monster, which are the Bloom equivalent of a Tuner Monster.
Each of those Seedling Monsters has a Bloom Condition that has to be fullified before that Monster can be used to Bloom Summon a Bloom Monster. Those Bloom Conditions have a great variety, raging from the need to destroy a Monster by battle to the need to control specific cards.
In Addition, most Bloom Monsters themself do also have a restriction, which Seedling Monster you can use to Bloom Summon them. They can range from different Types or Attributes, to Archetypes or even a specific Monster.
When Bloom Summoning, the Seedling Monster with the fullified condition is sent to the Graveyard and the Bloom Monster is placed from the Extra Deck on the field.
In any Fanfictions they might appear in, players use the term Blossomm for initiating a Bloom Summon (ex. I let my *name of Seedling Monster* blossom with *Bloom Condition* to Bloom Summon *name of Bloom Monster*), similar to Tune for Synchro Monsters and Overlay for Xyz Monsters. When this occurs, the Seedling Monster is covered in by a cocoon with its color corresponding its Attribute. If the Summoned Bloom Monster has an Attribute different from the Seedling Monster, the color of the cocoon changes, before it flakes of, revealing the newly summoned Bloom Monster.