Tried to do something different from my usual style here. It was challenging but I like the result.
The kanji in this idiom:
日 = day (or sun)
進 = progress
月 = month (or moon)
歩 = walking / steps
日進月歩 is a special kind of Japaneses idiom known as a 四字熟語 (よじじゅくご、yo-ji-juku-go), or 4-character idiom. As the name suggests, these are idiomatic phrases made of 4 kanji characters.
They often contain a small life lesson or moral message, similar to English sayings such as “you reap what you sow” and “the early bird gets the worm”.
This one is similar to the English saying “slow and steady wins the race”. 🐢