to rub
In German, the verb 'reiben' is used to express the action of rubbing something. It is a regular verb, which means that it follows the regular conjugation pattern for -en verbs. For example, 'Ich reibe die Tafel' means 'I am rubbing the blackboard'. The word 'reiben' can also be used in a figurative sense, for example, 'Das Problem lässt sich nicht so einfach wegreiben' means 'The problem can't be rubbed away so easily'.
We have to grate the apple
In this sentence, 'reiben' is used in the context of grating food. When you 'reiben' an apple, you're grating it to get small pieces.
I will rub my back against the wall
In this context, 'reiben' is used to express the act of rubbing one surface against another. When you 'reiben' your back against the wall, it means you are rubbing your back on the wall surface.
We must not rub against the grain
In this sentence, 'geben den Strich reiben' means to go against the grain or oppose the normal order of things. It is a metaphorical use of 'reiben' which literally means to rub or grate.