What does it mean to "strike while iron is hot"?
This is a historical figure of speech. It means to act quickly when the opportunity is still present.
Historically most people were familiar with the local blacksmith. The blacksmith would use coke a form of carbon to heat and partially melt the iron.
When the iron was hot enough it could be shaped into nails, horseshoes, shovels and other useful items. The blacksmith would strike the iron with hammers and tongs shaping the iron.
If the blacksmith waited to long the iron would cool and could not be shaped. Then the blacksmith would have to reheat the iron and start over.
Hence strike the iron when it is hot.
Don't wait take advantage of the opportunity while the opportunity still exists.