A suffix attached to the stem of a verb (the -masu stem) meaning 'difficult to ...' or 'hard to ...'.
この説明は専門用語が多くて分かりにくい。
This explanation contains many technical terms and is hard to understand.
古い字は暗くて読みにくい。
The old characters are faded and hard to read.
あの人とは気持ちを伝えにくい。
It's hard to convey my feelings to that person.
この靴は歩きにくいから、長時間は履けない。
These shoes are hard to walk in, so I can't wear them for long.