It can mean two things: what you foresee will happen (potentially probability-based), given factors like past experiences. It can also mean what you, or someone else wants or predicts to happen. Some similar words include "objectives" and "targets." Expectations tend to have a big effect on the feelings after the outcome occurs.
Means setting your expectations high, even if they are unlikely to be met. Schools often overemphasize this on students potentially to the point of burnout. One downside is that you may need to be prepared for the possibility of disappointment if you are using such an approach. However, one advantage can be that you won't need to change it constantly, which could prevent a sense of nihilism when you achieve a goal and have to set another one such that you continue to do work, like Sisyphus Rock. Another one is that it is also very socially attractive, meaning people love to hear it.
The opposite of the above. It helps keep your brain at peace. However, it might also gain laziness and be less efficient at handling tasks which may cut into your daily time management. And saying low expectations also implies laziness (socially speaking)), so be careful.
At the end of the day, if the metaexpectations of what level of expectations should be set is causing decision fatigue, as there is no correct answer due to it being subjective, one approach is to not even bother in the first place! Instead, you can do action each day that you think is helpful. Unlike SMART objectives, this means that there is no time pressure and helps avoid stress and chasing the impossible. Rather, a simplified approach is to just categorize things into good, neutral, and bad with your subjective judgement.
Still, the education system and businesses love to set expectations because they think it helps guide people. In reality, it can cause dissatisfaction among both parties, which can even reach guilt-tripping. As such, if they set a target for you, this is where coercion occurs often due to lack of agency. I'm sorry to say, but obey it, and act like you like it and will exert effort towards it. At worst you can negotiate. Your internal values can be seperate from external influences.
What if you don't have them: You can be free from stress to an extent of a thing not going out to plan. Of course this is not a command to give up on everything though.