The BHK full form is bedroom, hall and kitchen. It is used to convey the number of rooms in a property. For example, a 2BHK means that the particular property has two bedrooms, one hall and a kitchen. A 1 BHK means that the property has one bedroom, one hall and a kitchen.  3BHK means there are three bedrooms, a hall and a single kitchen in the property. Even if there are two bathrooms/toilets, sellers may or may not advertise it that way but will definitely mention the number of bedrooms.

In short,

1BHK apartment means 1 bedroom, hall, kitchen.

2BHK apartment means 2 bedrooms, hall, kitchen

3BHK apartment means 3 bedrooms, hall, kitchen

4BHK apartment means 4 bedrooms, hall, kitchen

Note that even if the abbreviation does not include a "T" as in a toilet space, all the BHK have a bath and toilet space. Some sellers may market their property as a 3BHK apartment +2T property. In this "2T" refers to two toilets. This is to highlight the benefit of the added space in the 3 BHK.