Explain to me the "Hit" event?

local x=Instance.new("Explosion")
x.Hit:connect(function(hit)
    hit.BrickColor=BrickColor.new("Really blue")
end)
x.Parent=workspace

Any part partially inside the BlastRadius will turn blue.

As any event, we use :connect() to connect a function to run when this event happens. This function is passed the arguments which are provided with the event.

According to the wiki, the Explosion.Hit event passes an Instance as first argument and a Number as second argument. The first argument is the part which is hit, the second is the distance from the explosion center to that part.

Example code: remove all red bricks when an explosion fires.

function FireExplosion(position)
    local new = Instance.new("Explosion")
    new.Hit:connect(function(part, distance)
        if part and part.BrickColor == BrickColor.Red() then
            part:Destroy()
        end
    end)
    new.Position = position or Vector3.new()
    new.Parent = game.Workspace
end

FireExplosion()