Ship conservation.
The goal is simple - to reduce the consumption of MSP on unused ships.
Conservated ships do not consume MSP (or consume x0.1) for maintenance and cannot follow orders.
During conservation, everything is unloaded from the ship - fuel, MSP, crew. Ship Maint is set to 2.0 if lower. Thus, it will take some time to return the ship to a combat-ready state - you can urgently activate several ships in a couple of hours to defend the colony, but they will not be very suitable for a long war.
And another serious limitation is Crew Grade and Training. They will decrease to Race Training Level and 0%. This will definitely not allow you to build 5,000 dreadnoughts and immediately send them to war. 
There are regular posts about ‘mothballing’ ships, but in many years we haven’t managed to find a solution that works. Maintenance is a huge part of the game, so there are too many advantages to effectively turning it off, but not enough disadvantages involved. If you want to propose detailed mechanics, so I can crunch the numbers, please create a separate post as it is a fairly popular topic.