The mattress width mentioned in the question indicates a size used in the United States on a cot frame; 30 inches wide by 74 inches long. In the United Kingdom, it is called a small single, 30 inches wide by 75 inches long. If the mattress is shorter, 30 inches wide by 70 inches long, it is called a cot or mini single in both countries. For reference, the standard width of a crib mattress is approximately 27 inches, also the same in both countries.

The standard twin size mattress in the United States is 39 inches wide by 75 inches long. In the United Kingdom, the standard twin size mattress

measures 36 inches wide by 75 inches long.

It's for a narrow single bed that's only 2'6" or 76cm wide. This could be a narrow bed or cabin or bunk bed. Normally they state if it's for a bunk bed as you need a firmer mattress to feel comfortable on the wooden slats of a bunk bed.

Its a bunk bed mattress... it's 2'6'' wide

It means it's only 2 and a half feet wide. Must be a crib mattress or something, because that's REALLY slender.