A crown (sometimes called a "cap") is a shell that your dentist makes to fit right over the top of a tooth, strengthening it.
Probably because a tooth has been weakened by extensive dental decay, large fillings, accidental damage or root canal treatment.
A crown holds together the remaining tooth and any dental filling material, making it stronger.
A crown can also be used to greatly improve the appearance of a tooth that is damaged, discoloured, mis-shapen or misaligned.
If your dentist has suggested a crown then this is probably because the tooth is weak.
It is unlikely that alternative treatments such as fillings, inlays or veneers would be effective (the tooth may well break after they have been placed)
If alternatives are viable in your case then your dentist will discuss the options with you.
Your dentist will shape the tooth by removing some of the outer surface, and sometimes "build up" the core of the tooth.
Next, they will take an "impression" (ie make a mould of) your teeth and take a measurement of how you bite together.
Your dentist will fit a temporary crown while the permanent crown is being made so that the tooth looks and feels the same between visits.
The impressions are then passed on to a specialist dental laboratory to prepare the crown and your dentist will see you for a second time to fit it using special dental cement.