You shouold trade money that you not afraid too loose. Thats the main rule of MM
Sometimes this becomes funny when read just once, because basically everyone will still be afraid of losing whatever money they use, even though that money is money that is not used elsewhere, they will still be afraid of losing it just like that, because of that. the name is money is always hard to get.
What it mean is do not invest the money that belongs to your saving or the money for your budget, remember to set aside the money for everyday needs, savings and for investment, you need to learn that in trading, the money you will invest is either make your money grow or make you lose, it is up to you to where and when will you use it, there's a difference between long and day trade.