Some tablets are designed to release the medicine slowly over hours or to have a protective coating, and crushing or cutting them can release the whole dose at once or destroy the coating. This can cause a sudden overdose, stomach irritation, or stop the medicine working properly.
Other tablets are scored and intended to be split, or can be crushed safely if you struggle to swallow them. The only reliable way to know which category your tablet falls into is to check the leaflet or ask your pharmacist.
If swallowing tablets is difficult, do not just start crushing them. Your pharmacist may be able to suggest a liquid form, a smaller tablet, a different medicine, or safe techniques to make swallowing easier.