The word "should" can be used when talking about an expected or probable outcome. Here are some examples:
"I posted the letter on Monday, so you should recieve it by Friday."
"It's already 9pm. John should have been here half an hour ago."
"According to the weather report, it should be a sunny day tomorrow."
"When the temperature approaches 100, we should see the water starting to boil"
It can also be used in judgments about what's right or wrong, good or bad, desirable or undesirable. These judgments can be moral / ethical:
"You should be respectful to your parents."
"Drunk drivers should be severely punished."
"You shouldn't keep the money that you found on the street. You should hand it to the police."
Or they can be aesthetic:
"It was an interesting film, but it should have been shorter."
"You played that song too quickly. It should have been more slow."
Or they can be about a person's own self-interest:
"The doctor says I should drink less coffee."
"I should go to the gym more often."
"I should be more kind to the people I work with."
"I should study harder, otherwise I won't pass the exam."
There are other uses of the word "should", but I think these are the main uses.