The most common meaning of ont is pain or hurt. You can use it to describe localized pain, in which case it is often used as part of a verb phrase, ha ont (i), which literally means 'to have pain in'.
So you might need to say: jag har ont i halsen (literally 'I have pain in the throat', but more accurately translated as 'I have a sore throat'), jag har ont i magen ('I have a stomachache'), or jag har ont i ryggen ('I have back pain/my back hurts'). You might also hear people use the verb få, for example jag fick ont i magen ('I got a pain in my stomach') which tends to refer to a pain that came on suddenly.
A similar phrase is göra ont, which could be translated literally as 'to do/make pain', but basically means 'to hurt'.
This can be used with a body part as the subject of the verb, for example halsen gör ont ('the throat makes pain', or 'my throat hurts'), or the subject might be the thing causing the pain. For example, det gör ont means 'it hurts' – and is the title of a catchy Swedish Eurovision entry from the early 2000s. It doesn't need to be a literal pain: the phrase det gör mig ont att säga means 'it pains me to say' or 'I'm sorry to say'.
In Swedish, you can say 'allt gör ont och jag dör' GIF: via GIPHY
Ont comes from the Old Norse adjective vándr (evil), and when used as a noun (ett ont) it means an evil or a bad thing. The related adjective ond also means evil, bad or angry, although it is a slightly old-fashioned term for anger. And look out for the set phrase på gott och ont (for better or worse).
Another use of ont you should be aware of is that when combined with the preposition om, it has a metaphorical sense, meaning 'lack of' or 'shortage of', and is most often used with uncountable nouns. You might hear someone say they have ont on tid (lack of time), ont om pengar (lack of money) or ont om vatten (lack of water).
Har du ont nånstans?
Do you have any pain?
Den här valpen är så gullig att det gör ont
This puppy is so cute it hurts