Having or pursuing multiple opportunities simultaneously.
I have been out of work for 6 months, but I have a number of irons in the fire.
Blacksmiths traditionally worked iron into shape by hammering. The iron being worked would be heated in the fire until it was red-hot and malleable. The Smith removes the iron from the fire and shapes it with repeated blows from a hammer. They need to work quickly before the iron cools. Once the iron is cool, it becomes brittle and cannot be hammered.
Once removed from the fire, the iron cools quickly. It takes longer to heat the iron to red-hot than it takes for it to cool. Blacksmiths work more efficiently by having multiple pieces of iron in the fire heating simultaneously. In that way, the Smith can always have a piece of iron red-hot and ready for hammering. The cooled piece would be returned to the fire if it needed more hammering.
Alternative : Refers to the number of irons (think pressed shirts here) a colonial woman kept hot in the fire at one time on washing day. Irons in those days were of course not powered by electricity, they were heated by fire. Irons made from iron took a long time to heat up in the fireplace so women would often have several to speed up their ironing chores.