Ugh, I had written a lengthy post and lost it. I haven't played Widelands itself much, but I have played Settlers 2 a lot. If anything I explain below is different in Widelands, please correct me ;)
Anyway, a TLDR for those that have no idea of what to expect: The Settlers 2 is a lot more about logistics and planning use of space than anything else. It needs very little input most of the time, it's a very slow paced game.
The main things to keep in mind is available space, roads and resources. The game is separated in hex-ish tiles. Depending on how much free space there is, you can build a basic, medium or large building. As a rule of thumb, basic buildings require no resources to function (one exception being mines, which need food); medium buildings receive either 1 or 2 resources and deliver the worked result; large buildings are usually farms or a fortress.
So, imagine Age of Empires 1, but if you had to connect every building with a road network, with every worker and every resource traveling through it, one at a time. Once you set up something to be built, you'll see a worker walking his way there, as well as resources being carried towards it. The busiest roads can receive a donkey that will also haul resources between the connected flags - you cannot manually upgrade roads, even if you have a surplus of donkeys.
Unlike AoE or pretty much every RTS, you don't train units at all. You need a minimal military to garrison military buildings, which will increase your borders. Once any of these is fully built, a number of soldiers will come out of HQ and move to occupy it. HOWEVER, if you are attacked, only the soldiers within that building will protect it. You don't participate in combat at all. The soldiers just line up and fight. When a military building is occupied by the enemy, it and everything that was within the lost border is destroyed.
That'd sinful!