The symbol is a generic mana symbol. It represents a cost that can be paid by one mana that was produced by a snow source. That mana can be any color or colorless.

Unlike effects that care only whether or not you spent snow mana to cast a spell, Boreal Outrider cares about the color of that snow mana. It doesn’t matter which part of the total cost to cast the creature spell the snow mana was spent on, only that the spent snow mana matches one of the creature spell’s colors. For example, if a red creature spell has an additional cost to cast it and you spend red mana from a snow source to pay that cost, Boreal Outrider’s ability will trigger.

A creature affected by Boreal Outrider’s triggered ability will enter with only one additional +1/+1 counter on it, even if you spent more than one snow mana of the appropriate color(s) to cast it.

The Kaldheim set doesn’t have any cards with mana costs that include , but some previous sets do. If an effect says such a spell costs less to cast, that reduction doesn’t apply to any costs. This is also true for activated abilities that include in their activation costs and effects that reduce those costs.

Snow isn’t a type of mana. If an effect says you may spend mana as though it were any type, you can’t pay for using mana that wasn’t produced by a snow source.

If you control multiple Boreal Outriders, each of their triggered abilities will trigger separately. A creature could enter the battlefield with multiple +1/+1 counters on it this way, even if you spent only a single snow mana of the appropriate color(s) to cast the creature spell.

Some cards have additional effects for each spent to cast them. You can cast these spells even if you don’t spend any snow mana to cast them; their additional effects simply won’t do anything.

Snow is a supertype, not a card type. It has no rules meaning or function by itself, but spells and abilities may refer to it.