Sock liners are made of a thin polyester or lightweight wool material that help wick sweat away from your feet. You wear them under your thicker hiking sock and are mainly recommended as a preventive measure to getting blisters.
Wearing sock liners on a hiking vacation is definitely a matter of personal preference, and they do not work for everyone. The main problem with sock liners is that it can be challenging to find the right thickness sock to go with the right thinness of liner. If the end result of 2 socks is that they are too fat for your boot, well, you are creating a bigger problem and are likely to get blisters.
I used to be an avid sock liner wearer - I had the right weight wool sock and the right weight liner and it was beautiful. But those socks got holes, and they stopped manufacturing them to replace them. I tried numerous sock combinations with and without liners and came to the conclusion that liners made my feet more prone to blisters. I thought I had my system figured out: I was one of those people for whom liners did not work- until I went on a backpacking trip in Havasu Canyon a couple years ago.
My no-liner system failed me and I got bad blisters. After 3 days in the Canyon, hiking mostly in Chaco sandals and soaking my feet in cool water, the blisters diminished. Time came to backpack out (10 miles) and I went back and forth on whether to wear the liners on the way out. But they really made my boots tight and I worried I would get blisters in new areas. So I decided to go just with hiking socks again. 2 miles into it my feet were killing me. I stopped, put on moleskin and my sock liners. I hiked the remaining 8 miles, got no new blisters and the blisters I had got no worse. I was amazed. I think my feet were just too hot and needed the liner as both a cooling effect and to reduce the friction.
Whatever the reason - it worked. Now, not only do I advocate testing whether or not sock liners work for you, but test whether they work for you in different climates, on different hikes, different days. I am now a sometimes sock liner wearer.