。
You have to be faithful to your promises.
Literal
Promise [to-には] faithful must-[must-なくてはいけない].
忠実 is a na-adjective ('faithful, loyal'), and with the target marked by には it reads 'be faithful with respect to promises.' The obligation pattern ~なくてはいけない is another variant in the 'must' family, formed from the negative -te + wa + ikenai. Literary/moral register.