The following are just bits I pinched from the internet.
The phrase "Lest we forget" is commonly used in remembrance services and commemorative occasions in English speaking countries, in particular Remembrance Day and ANZAC Day.
"Lest" means something like "in case". For example, "We should close the door,lest the dog escapes" means, "We should close the door in case the dog escapes [through the open door]". The phrase "lest we forget" is most often used in ceremonies to commemorate wars, such as Remembrance Day in Canada.
Lest We Forget may refer to: "Lest We Forget (phrase)", a phrase in the poem "Recessional" by Rudyard Kipling. "Ode of Remembrance", United Kingdom, Canada, Australia, and New Zealand.