Ideas are the precursors to Knowledge.
Knowledges begin as ideas thought up by the population. Ideas require certain conditions to be met before the population can come up with them. For example, a territory that is experiencing a water shortage after having exhausted Water Collector placement may come up with the Well Digging idea. Some ideas require certain buildings to built. Cast nets for example require 2 Fisherpigs to be built. Most military ideas require you to have a number of precursor units recruited, for example, Archery requires around 30 Javelineers.
Outside of rewards from completing early game missions, ideas always happen automatically. Ideas have a random chance to be thought up once conditions are met. This chance is higher for porcos with higher intelligence, and some ideas will not occur if your porcos' intelligence is too low.
Once an idea is researched it becomes a Knowledge. After a knowledge is acquired, it can be traded to other players as an idea. Ideas traded this way allow players to research knowledges even if they don't meet the conditions to come up with the idea themselves. Players can also spend Research Points in order to gift knowledges directly, skipping the need for the receiver to research it themselves, although only knowledges which have all their prerequisites met can be traded in this way.
It is very likely that ideas like Bronze working and Iron working are time gated in persistent servers to allow for enough play time for each age.