BackupBackup is a triggered ability. “Backup N” means “When this creature enters, put N +1/+1 counters on target creature. If that’s another creature, it also gains the non-backup abilities of this creature printed below this one until end of turn.” Cards with backup have one or more abilities printed after the backup ability. (Some cards with backup also have abilities printed before the backup ability.)
If a permanent enters the battlefield as a copy of a permanent with a backup ability or a token is created that is a copy of that permanent, the order of abilities printed on it is maintained.
Only abilities printed on the object with backup are granted by its backup ability. Any abilities gained by a permanent, whether due to a copy effect, an effect that grants an ability to a permanent, or an effect that creates a token with certain abilities, are not granted by a backup ability.
The abilities that a backup ability grants are determined as the ability is put on the stack. They won’t change if the permanent with backup loses any abilities after the ability is put on the stack but before it resolves. (CR 702.165) 1 (When this creature enters, put a +1/+1
counterTo counter a spell or ability is to cancel it: remove it from the stack without resolving it. A counter on an object or player is instead a marker that changes characteristics or interacts with rules or abilities; for example, a +1/+1 counter modifies a creature’s power and toughness. (CR 701.6; 122.1) on target creature. If that's another creature, it gains the following ability until end of turn.)
Whenever this creature attacks,
doubleDoubling a creature’s power and/or toughness creates a continuous effect. This effect modifies that creature’s power and/or toughness but doesn’t set those characteristics to a specific value. See rule 613.4c.
To double a creature’s power, that creature gets +X/+0, where X is that creature’s power as the spell or ability that doubles its power resolves. Similarly, an effect that doubles a creature’s toughness gives it +0/+X, where X is that creature’s toughness. Doubling a creature’s power and toughness gives it +X/+Y, where X is its power and Y is its toughness.
If a creature’s power is less than 0 when it’s doubled, doubling that creature’s power instead means that the creature gets -X/-0, where X is the difference between 0 and its power. Similarly, if its toughness is less than 0 when doubled, it gets -0/-X. If one characteristic’s value is negative but the other isn’t when both are doubled, it gets -X/+Y or +X/-Y, as appropriate.
To double a player’s life total, the player gains or loses an amount of life such that their new life total is twice its current value.
To double the number of a kind of counters on a player or permanent, give that player or permanent as many of those counters as that player or permanent already has.
To double the amount of a type of mana in a player’s mana pool, that player adds an amount of mana of that type equal to the amount they already have.
To double an amount of damage a source would deal, that source instead deals twice that much damage. This is a replacement effect. (CR 701.10) the number of +1/+1
countersTo counter a spell or ability is to cancel it: remove it from the stack without resolving it. A counter on an object or player is instead a marker that changes characteristics or interacts with rules or abilities; for example, a +1/+1 counter modifies a creature’s power and toughness. (CR 701.6; 122.1) on target creature. That creature can't be blocked by creatures with power 2 or less this turn.