Unfortunately as with many legal concepts the simplicity of a basic assault gets complicated because life is complicated.
First, as a technical matter an assault takes place whenever you touch someone without their consent. So even putting your hand on someone's shoulder and telling them off is an assault. Such an assault may be trivial - but it is an assault.
Second, consent is not always straightforward.
Consent means an agreement to the touching. Agreement requires knowing what is happening - you have to be conscious and making the decision to consent. So you can, obviously, consent to having sex. But sex without consent amounts to very serious sexual assault. You can consent to a fight - it's a bad idea but it's legal. However you (probably) cannot consent to a fight leading to a lasting injury or death. So a "fight to the death" can never be consented to as lawful.
In a hockey game people consent to being checked - but what about illegal body checking? Generally an illegal check isn't an assault but it all depends on the players, what they agreed to and how the game is usually played.
Similarly, someone may consent to being kissed but not to anything else - and absent consent sexual activity almost always amounts to assault.
Now, the law accepts that there is an assumed consent to certain touching - say tapping someone gently on the shoulder to get their attention. Similarly, if you find someone injured and unconscious you can give them emergency treatment - consent is assumed.
But if the injured person wakes up and says stop you must stop. And if you know someone does not want to have their shoulder tapped you shouldn't tap their shoulder.
Third, sometimes applying force without consent is legally justified. For example a parent may use moderate and reasonable force to discipline a child. Similarly if someone attacks you it is ok to defend yourself using only the reasonable force needed for self protection. Of course what is moderate and reasonable is something people can disagree about and that can lead to all manner of legal argument.
What all this boils down to is a recognition that everyone has the right to physical integrity. You cannot touch someone without their permission. So long as you behave respectfully to others you should be fine; but if you step over that line you may be guilty of an assault.