What does the For Loop in this swimming script do and what is the limitations on BodyPositioning?

for i = 1, #c do
                if c[i].Name == "SwimForce" then
                    c[i]:Destroy()

Question below

I'm working on a swimming script for my game and so I decided to look off of others work to get an idea on how to create one. I finally came across one that worked and I was looking over the script and then I came to this For loop. I don't understand what it does at all.

The whole basis of the script is to create a BodyPosition and place it into an R15 model character. The values for the BodyPosition is already predefined and all there is to do is place it into a character; which is handled by a While loop to check if the BodyPosition is currently in the Torso of the character and if so, what it'll do while it's in the character.

Here is the full script:

waterlevel = -3     -- Water level, in terms of height

repeat wait() until game.Players.LocalPlayer.Character ~= nil
local char = game.Players.LocalPlayer.Character

function makeswim()
    if char:FindFirstChild("SwimForce") == nil then
        local bp = Instance.new("BodyPosition")
        bp.Name = "SwimForce"
        bp.maxForce = Vector3.new(0,85000,0)
        bp.position = Vector3.new(0,waterlevel + 3,0)
        bp.Parent = char.LowerTorso
    end
end


while true do
    if char.LowerTorso.Position.Y < waterlevel then
        if char.LowerTorso:FindFirstChild("SwimForce")==nil then
            makeswim()
        end
    else
        if char.LowerTorso:FindFirstChild("SwimForce") then
            local c = char.LowerTorso:GetChildren()
            for i = 1, #c do
                if c[i].Name == "SwimForce" then
                    c[i]:Destroy()
                end
            end
        end
    end
wait(0.1)
end

The problem that I'm having is what does the For loop do? I didn't clearly understand why there was a hashtag in place for a number, for i = 1, #c do, and why you couldn't do, if i.Name == "SwimForce" then i:destroy, instead of, if c[i].Name == "SwimForce" then c[i]:Destroy().

Lastly, how is it that I'm able to move with a BodyPosition in my character when the script above is running, but whenever I place it into my character manually I can't?

Thanks for the help!