Look up glp plotter it will show you estimated blood levels of the drug over time on a graph so it is easier to understand.
The reason you are supposed to increase doses slowly is to avoid getting hit by sudden severe side effects, around 1% will need hospital treatment from starting or increasing doses of GLPs due to dehydration from nausea and vomiting. This is what you want to avoid. Not the only side effect, but the most common serious one.
After a dose it take about 24 hours for it to be absorbed and reach peak levels in your blood , this is typically when the worst side effects will happen. Then it takes 6 days to drop to half that level, this is relevant because if you do get severe side effects or even less bad ones it can take a week for them to settle down. Each dose you have after that builds up in your blood stream and it takes about 4 weeks of weekly doses to get to the maximum level, so you can get worse side effects several weeks after a dose increase. This is why you are meant to wait 4 weeks to increase doses, as that is how long it takes to get to the new steady state blood level from the previous dose increase.
The standard recipe worked out by the company that made the drug is designed to have a fairly low risk of severe sudden side effects, which is start at 2mg and increase by 2mg every 4 weeks after that until you get to 12mg. If you are not severely overweight you may not need doses that high, if you have a BMI of 35+ you probably will.
For reta the common side effects are nausea and vomiting and constipation and diarrhoea, also skin sensory symptoms are fairly common and some people notice a increase in heart rate.
The odds are you will get some side effects at some stage, but there is a lot of individual variation in where this happens, one person might start to get nausea at 10mg and another might vomit for a week at 4mg, and about half will not get any nausea. The useful bit of advice I can give is adjust the dose increases based on the effects on you, If you get good appetite suppression at lower doses you may not need to keep increasing doses, if you get mild nausea at one dose the odds are it will get worse when you increase the dose so that might be a good time to slow down on increasing the doses.