there is some explanation here
http://www.expat-finland.com/pdf/driving_in_finland.pdf
The rule is that YOU give way to traffic coming from public roads on YOUR RIGHT, except if your road is a "priority road" or if the road on the right has a give way sign.
This in effect is a totally stupid way to design roads. you often have to look for THE BACK of a give way sign to see if cars coming from the right have a give way sign. if the angle is right (it's directly edge on to you) then it's almost invisible. You eventually learn the junctions on your regular route (great logic there, you have to learn through experience how different junctions are posted). Because of this people will stop unnecessarily in front of you since they don't know they have priority.
"priority road" is one with the speed limit > 40km/h or a yellow diamond sign (page 23 of the pdf). In Holland they have these yellow diamonds just before the junctions where you have priority - very sensible. In Finland they save their money and avoid too many signs, so you must remember there was one a few hundred metres earlier (more great logic). There is no logic as to which roads are "priority roads" and which aren't. I know of several stretches of road in helsinki where the road will change from normal to "priority road" although there is no difference in road width/volume/road type/speed limit.
To complete the disaster, you must be able to distinguish between public roads and private roads/driveways. Since only public roads have priority from the right. Sounds sensible but many shop entrances, big driveways or farm roads look like public roads.I suppose you can look for a street name sign - since private roads don't normally have those.
All in all traffic chaos. but most places have little traffic so the planners don't bother with the concept of getting maximum number of cars safely along the roads. Why bother with clear signs if there is one car every 10minutes!
What about the cars that are already on the road ?. Do they speed down or up when they see some car trying to merge or do they usually change the lane
If you are talking about motorways then the cars joining SHOULD adjust their speed and fit into the gaps like already mentioned here. In reality people drive on at full speed expecting a gap. most drivers already on the motorway will move to the left to let these impolite types join. some drivers (older types) will actually slow down to let people join - so affecting traffic behind them. if you are joining the motorway and behaving properly you should not pull directly in front of cars already on the motorway so these well intending types are actually a problem since you slow down, they slow down and soon you'll both be doing 30km'h on the motorway! these same types will stop while going around a roundabout to let people join from the right (something I have never ever seen happen in England). Watch out for these types (normally older drivers) since you will rear end them otherwise. They are also likely to drive straight onto roundabouts and not even look to their left, so watch out for this situation also.
After a few years you'll get to identify the mistakes people are about to make. pull in front of you and force you to brake, come onto motorways and roundabouts incorrectly. change lanes without looking. most of them you can avoid. probably the people going through red traffic lights are the hardest to avoid since they are going so fast (most of these are on the phone while doing it). But still when driving in finland I have to brake to avoid somebody behaving impolitely at least once an hour while driving. In England I might go a week without having to brake for somebody driving impolitely.
. If not, one should watch whether there's already vehicles on the road or approaching closer. They have right of way.