Fractal is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

How it works : Fractal

Original English text — Wizards of the Coast / Scryfall

No additional ruling has been published for this card.

Interactions with other cards

These cards' text mentions Fractal.

Deekah, Fractal Theorist

Magecraft — Whenever you cast or copy an instant or sorcery spell, create a 0/0 green and blue Fractal creature token. Put X +1/+1 counters on it, where X is that spell's mana value. {3}{U}: Target creature token can't be blocked this turn.

Fractal Harness

When this Equipment enters, create a 0/0 green and blue Fractal creature token. Put X +1/+1 counters on it and attach this Equipment to it. Whenever equipped creature attacks, double the number of +1/+1 counters on it. Equip {2}

Oversimplify

Exile all creatures. Each player creates a 0/0 green and blue Fractal creature token and puts a number of +1/+1 counters on it equal to the total power of creatures they controlled that were exiled this way.

Primo, the Unbounded

Trample Primo enters with twice X +1/+1 counters on it. Whenever one or more creatures you control with base power 0 deal combat damage to a player, create a 0/0 green and blue Fractal creature token. Put a number of +1/+1 counters on it equal to the damage dealt.

Emil, Vastlands Roamer

Creatures you control with +1/+1 counters on them have trample. {4}{G}, {T}: Create a 0/0 green and blue Fractal creature token. Put X +1/+1 counters on it, where X is the number of differently named lands you control.

Berta, Wise Extrapolator

Increment (Whenever you cast a spell, if the amount of mana you spent is greater than this creature's power or toughness, put a +1/+1 counter on this creature.) Whenever one or more +1/+1 counters are put on Berta, add one mana of any color. {X}, {T}: Create a 0/0 green and blue Fractal creature token and put X +1/+1 counters on it.

Lattice Library

This enchantment enters with X study counters on it. When this enchantment enters and whenever you cast your first spell with {X} in its mana cost each turn, create a 0/0 green and blue Fractal creature token. Put a number of +1/+1 counters on it equal to the number of study counters on this enchantment.

Paradox Zone

This enchantment enters with a growth counter on it. At the beginning of your end step, double the number of growth counters on this enchantment. Then create a 0/0 green and blue Fractal creature token. Put X +1/+1 counters on it, where X is the number of growth counters on this enchantment.

Body of Research

Create a 0/0 green and blue Fractal creature token. Put X +1/+1 counters on it, where X is the number of cards in your library.

Kasmina, Enigma Sage

Each other planeswalker you control has the loyalty abilities of Kasmina. +2: Scry 1. −X: Create a 0/0 green and blue Fractal creature token. Put X +1/+1 counters on it. −8: Search your library for an instant or sorcery card that shares a color with this planeswalker, exile that card, then shuffle. You may cast that card without paying its mana cost.

Emergent Sequence

Search your library for a basic land card, put it onto the battlefield tapped, then shuffle. That land becomes a 0/0 green and blue Fractal creature that's still a land. Put a +1/+1 counter on it for each land you had enter the battlefield under your control this turn.

Zimone, All-Questioning

At the beginning of your end step, if a land entered the battlefield under your control this turn and you control a prime number of lands, create Primo, the Indivisible, a legendary 0/0 green and blue Fractal creature token, then put that many +1/+1 counters on it. (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, and 31 are prime numbers.)