What is the motto of the RAF; and, what does it mean?

Latin: "Per Ardua ad Astra"
English "Through Adversity to the Stars"

The authoritative translation of the motto is just as uncertain as the source. Since there can be a number of different meanings to "Ardua" and "Astra", scholars have declared it to be untranslatable. To the Royal Air Force and the other Commonwealth air forces, however, it will remain translated as "Through Adversity to the Stars".

https://en.wikipedia.org/wiki/Per_ardua_ad_astra

Colonel Frederick Sykes, who was the first Commanding Officer of the RFC, asked his officers to come up with a motto for the new service. The service was called the "Royal Flying Corps".

JS Yule came up with "Per Ardua ad Astra", which translated to "Through Struggles to the Stars". Colonel Sykes approved, submitted it to the War Office and it was then submitted to the King. Once the King approved, they had a motto of which to be proud.

https://raf-waddington.com/history/hrafmotto.html
raf.mod.uk/history/theroyalairforcemotto.cfm no longer exists