What do we mean when we say "he has many irons in the fire"?

He is always ready for whatever comes; because he is preparing spare tools (in the metaphorical sense) and not just waiting around for what demands that he makes one. 

It means having too many things to do at the same time.  It comes from blacksmiths putting bars of iron in the fire to get them hot enough to work with.  The smith has to be aware of the status of the iron.  If he or she has too many to keep track of, it could be a big problem. 

 

Doing many things or activities at same time.

to be involved in several jobs or activities at the same time

To have many activities at the same time. To have many options or courses of action.

A number of jobs or possibilities available at the same time. It is recommended to keep several irons in the fire when you’re looking for work.