Cfrgtkky

The Path of the Ancestral Guardian barbarian's Ancestral Protectors feature (XGtE, p. 10) says:

While you’re raging, the first creature you hit with an attack on your turn becomes the target of the warriors, which hinder its attacks. Until the start of your next turn, [...] when the target hits a creature other than you with an attack, that creature has resistance to the damage dealt by the attack.

Let's say the barbarian hits a wizard, and after the wizard casts a fireball into the party (or even a toll the dead on an individual besides the barbarian).

Do the party members get resistance to the damage? Or because it specifies "attack", and a fireball (and toll the dead) is not described as an attack, do they take full damage?

The party takes full damage from both spells

Unless the spell says you are making an attack roll or labels itself an attack you are not attacking.

If there’s ever any question whether something you’re doing counts as an attack, the rule is simple: if you’re making an attack roll, you’re making an attack. (PHB 194)

Fireball says:

Each creature in a 20-foot-radius sphere centered on that point must make a Dexterity saving throw.

This is a saving throw made by the affected creatures, not an attack roll. Thus it is not an attack.

The same is true of toll the dead which says:

The target must succeed on a Wisdom saving throw or take 1d8 necrotic damage.

Ancestral Protectors specifically requires an attack:

When the target hits a creature other than you with an attack...

Thus, the party will get no protection from a fireball or toll the dead since they are not attacks.

The fireball does not count as an attack because there is no attack roll; it simply calls for a (Dexterity) saving throw. The same is true for toll the dead:

The target must succeed on a Wisdom saving throw

Therefore, the Barbarian's ability does not apply.