+X means to add that many counters.
-X means to remove that many counters. An ability with a negative loyalty cost can't be activated unless the planeswalker has at least that many loyalty counters on it.
0 means to perform no modifications.
- Loyalty Ability
- An activated ability with a loyalty symbol in its cost. See rule 606, “Loyalty Abilities.”
- 107.7. Each activated ability of a planeswalker has a loyalty symbol in its cost. Positive loyalty symbols point upward and feature a plus sign followed by a number. Negative loyalty symbols point downward and feature a minus sign followed by a number or an X. Neutral loyalty symbols don’t point in either direction and feature a 0. [+N] means “Put N loyalty counters on this permanent,” [-N] means “Remove N loyalty counters from this permanent,” and  means “Put zero loyalty counters on this permanent.” Loyalty symbols may also appear in abilities that modify loyalty costs.
- 606. Loyalty Abilities
- 606.1. Some activated abilities are loyalty abilities, which are subject to special rules.
- 606.2. An activated ability with a loyalty symbol in its cost is a loyalty ability. Normally, only planeswalkers have loyalty abilities.
- 606.3. A player may activate a loyalty ability of a permanent they control any time they have priority and the stack is empty during a main phase of their turn, but only if no player has previously activated a loyalty ability of that permanent that turn.
- 606.4. The cost to activate a loyalty ability of a permanent is to put on or remove from that permanent a certain number of loyalty counters, as shown by the loyalty symbol in the ability’s cost. This cost may be modified by other effects.
- 606.5. If the total cost to activate a loyalty ability contains multiple costs to add or remove loyalty counters, those costs are combined into a single cost to add or remove loyalty counters, as appropriate.
Example: A player controls Carth the Lion, which says, in part, “Planeswalkers’ loyalty abilities you control cost an additional [+1] to activate. That player also controls a planeswalker with three loyalty counters. To activate one of that planeswalker’s abilities that normally costs [+1], they put two loyalty counters on it. To activate one of its abilities that normally costs [−4], they remove three loyalty counters from it.
- 606.6. A loyalty ability with a negative loyalty cost, taking into account any additional costs, can’t be activated unless the permanent has at least that many loyalty counters on it.