What is a fee market? Does it mean that blocks should be full, esp. after several halvenings?

As we all know, Bitcoin had experienced an unprecedented long-lasting congestion period in 2017. This seems quite horrible to some people generally, but it seems that such phenomenon was on the contrary considered as a positive sign by developers, for example:

Personally, I’m pulling out the champaign that market behaviour is indeed producing activity levels that can pay for security without inflation, and also producing fee paying backlogs needed to stabilize consensus progress as the subsidy declines.

I’ve heard that is about a strategy called “stable fee market”(might be inaccurate, sorry). So, I’m courious that does that mean the blocks should be kept full in order to keep Bitcoin safe? Does it merely a compensation of declining block subsidy? Is there any deeper reason behind?

Also, I wonder how could Replace-By-Fee take effects?