'Graft' was introduced in the Ravnica block, it is on numerous cards.

702.55. Graft

• 702.55a. Graft represents both a static ability and a triggered ability. "Graft N" means "This permanent enters the battlefield with N +1/+1 counters on it" and "Whenever another creature enters the battlefield, if this permanent has a +1/+1 counter on it, you may move a +1/+1 counter from this permanent onto that creature."
• 702.55b. If a creature has multiple instances of graft, each one works separately.

Translation:

• 702.55a. - This is a two parter. The first part is the rules to make the creatures bigger. The second defines the triggering ability for other Graft creatures.
• 702.55b. - Clarifying that Graft does stack.