This is the 75th most frequent Japanese word.
長
Chief; head; leader; long.
Here, '長' (cho) is used as a suffix to indicate a leader or head of a specific group, department ('部'; bu) in this case, forming '部長' (bucho).
彼は部長に昇進した。
He was promoted to department manager.
'長い' (nagai) is the adjectival form of '長' (naga), meaning 'long', and is often used to describe lengths or durations.
この川は非常に長い。
This river is very long.
As a title, '長' (cho) follows '会' (kai; meeting, association) to form '会長' (kaicho), indicating the president or chairperson of the association.