Boost. What does "Minimum Cache Lifetime" mean? In Boost module setting, there are 

Maximum Cache Lifetime 
and 
Minimum Cache Lifetime


settings. 



"Maximum Cache Lifetime" is like re-generate the page after that amount of time. 

How about "Minimum Cache Lifetime", what does it do? 

Updated:

Just a copy / paste from boost handbook for a reference. 

Max: Default is 1 hour and can be increased for sites that do not change very much. The maximum cache lifetime is the maximum amount of time that will elapse before the cache is emptied. Cache lifetime gets checked on cron runs. Flushing the content before it is expired can happen, for example when a node is edited.

Min: [&lt; none>] (select box) none is fine for small sites.
On high-traffic sites, it may be necessary to enforce a minimum cache lifetime. The minimum cache lifetime is the minimum amount of time that will elapse before the cache is emptied and recreated, and is applied to both page and block caches. A larger minimum cache lifetime offers better performance, but users will not see new content for a longer period of time.
Check out this handbook page describing all settings (for 6.x but still relevant to 7.x):

"On high-traffic sites, it may be necessary to enforce a minimum cache lifetime. The minimum cache lifetime is the minimum amount of time that will elapse before the cache is emptied and recreated, and is applied to both page and block caches. A larger minimum cache lifetime offers better performance, but users will not see new content for a longer period of time."