Semi-weekly is an adjective. It is used to describe something that is issued or happening twice a week. A thing that is described by this adjective is termed 'semi-weekly'. 'Semi-weeklies' is plural of semi-weekly. For example: he subscribed for a semi-weekly. It means he subscribed for something (especially a magazine or newspaper). This person receives this issue twice in a week.
Semi-weekly could also mean bi-weekly. Bi-weekly implies something that is issued or occurring twice a week. Bi-weekly could be used to refer to something that is issued or occurring once in two weeks. Thus, use of bi-weekly for semi-weekly is a bit confusing. Hence, semi-weekly as an adjective could mean bi-weekly; but, semi-weekly as a noun does not mean semi-weekly.
Semi-weekly could also mean bi-weekly. Bi-weekly implies something that is issued or occurring twice a week. Bi-weekly could be used to refer to something that is issued or occurring once in two weeks. Thus, use of bi-weekly for semi-weekly is a bit confusing. Hence, semi-weekly as an adjective could mean bi-weekly; but, semi-weekly as a noun does not mean semi-weekly.