adverb
now accepting; currently open for applications
Used on signs, websites, or announcements to indicate that applications, reservations, or submissions are being accepted at this time.
ただいま参加者を受付中です。
We are currently accepting participants.
ホームページには「受付中」と表示されていた。
The website displayed 'Now accepting applications'.
受付 is a noun meaning reception desk or the act of accepting; 受付中 is the adverbial form indicating the ongoing state of accepting.
Compound of 受付 (uke-tsuke, 'accepting') + 中 (chū, 'in the process of').