Aggro Token and the First Activation Token
For the core set, enemies will either prioritise the player with the aggro token or the closest player.
The skull icon represents aggro. The circle represents closest player. See the Enemy Activations section in the rule book (pg 24) for more details.
The first activation token is just used to identify who will go first at the next encounter based on the fact that they were next in line when the previous encounter ended.
From the official rules:
First Activation

No matter whether the encounter ends in victory or defeat, give the
First Activation token to the player who is next in turn order after
the last player who activated their character. The player with the
First Activation token will activate their character first in the next
encounter.

Aggro Token (emphasis added)

When a character starts their activation:
They gain 2 Stamina.
They gain the Aggro token.
They may swap items between their backup slot and hand slots (see ‘Equipment Cards’, p. 12).

The Basics p.19 (emphasis added)

Once you’ve set up the enemies, terrain, and traps, place the
character models on the entry nodes beside the door aligned
with the tile the party moved from. Remember that a node
cannot contain more than three models.
Next, choose which character led the way into the encounter
and place the Aggro token on that character’s model.
Then the battle begins!

However, when you first enter any encounter you may determine which player 'lead the way' independent of the first activation and give the Aggro Token to that player.
Note that for the first encounter, the players decide as a group which player goes first. After that play goes around the table in a clockwise fashion.

During the very first character activation, the players can activate
the character of their choice. From then on, the order in which
characters activate goes clockwise around the table.