Everyone understands the principle that not advancing means retreating.
Tips
history
Attested in the pre-Qin philosophical fragment 《邓析子·无厚篇》: 不进则退,不喜则忧,不得则亡 — a Warring States-era Legalist meditation on the binary of motion. The parallel 逆水行舟,不进则退 ('rowing upstream, no advance means retreat') made it into every modern classroom.
grammar
The 则 is a classical conditional: 'if... then...' Pattern: 不A则B = 'if not A, then B.' Other familiar examples: 不破不立 (without destroying, nothing is built), 不鸣则已 (if silent, then silent).