If you are going to miss a board meeting then you should send your regrets as soon as possible. Sending regrets allows the organization to determine if there is a risk quorum will not be achieved. If an organization gets regrets from a couple of board members the organization can contact the remaining board members and confirm their attendance.
If you do not send regrets then you are absent and that fact will be documented in the minutes. You may also be judged. Particularly if there is a problem with quorum, which might have been avoided if you had conveyed your regrets rather than simply failing to show up.
Whether you send regrets or not – you are not at the meeting and that fact will be taken into account when compliance with board meeting attendance policies is being determined.