Already read. Commonly refers to a message or email that the recipient has seen (antonym: 未読 'unread').
LINEで既読が付いているのに返信が来ないと気になる。
It bothers me when a LINE message shows as read but there's no reply.
メールを既読にした後で、後で返事を書こうと思った。
After marking the email as read, I thought I'd write a reply later.
既読スルーは相手に無視されている感じがして嫌だ。
Being 'read and ignored' feels like being ignored by the other person and I don't like it.