How is the pronoun 'on' used in French?

Words like I, you, she and they are all personal pronouns - this means that they take the place of nouns in a sentence to tell us who is doing an action, or is having an action done to them. We use them very frequently because they are a quick way to refer to people without always repeating things like “Jane”, “Dr Smith” or “the man in the green jacket”. 
Usually, pronouns in English can be directly translated into French - for example, ‘me’ means ‘moi’ and ’she’ means ‘elle’. However, this is not always the case - one case that you may already know about is the distinction between ‘tu’ and ‘vous’ in French, where we always say ‘you’ in English. ‘On’ is an even tricker pronoun, as it can be translated into English in several different ways, but it will make your French will seem much more natural if you can get the hang of using it.
The key to understanding the word ‘on’ is to remember that it is actually quite impersonal, considering it is called a personal pronoun! This is because it is used to refer to groups of people which are vague and undefined. Consider the following English sentences:
“They’ve discovered a new planet.”“We can’t keep polluting the atmosphere like this.”“People really don’t like tourists who don’t respect the local culture.”
In these sentences, who do we really mean when we say “they”, “we” or “people”? It isn’t exactly clear. It could be a group of experts, a majority of people in one area, a lot of people in general, or even the entire human race! The point is that the sentence leaves the details implied - it doesn’t say explicitly “who” is doing the action, and the listener is expected to understand from context. Ιn these cases, you should usually use the word “on” in French.
“They’ve discovered a new planet.” - “On a découvert une nouvelle planète.”“We can’t keep polluting the oceans like this.” - “On ne peut pas continuer à polluer ainsi l’atmosphère.”“People really don’t like tourists who don’t respect the local culture” - “On n’aime pas du tout les touristes qui ne respectent pas la culture locale.”
The most important thing is to be able to recognise the word 'on' and understand how it works - once you begin seeing the word in context, you will probably begin to get a feel for it. Don't worry if you don't yet feel comfortable using it, as you will be understood just fine if you phrase sentences like this in other ways.