Meaning: Respect and love others.
Significance:Ren is the key idea of Confucianism which has dominated a feudal society that in essence has lasted 2000 years and for that reason its influence over the history, social structure and the people of China cannot be overlooked.

BTW,the Confucianism was formed in the spring and autumn period.
see here https://www.travelchinaguide.com/intro/religion/confucianism/

How the period of warring states gave rise to new ideas?

The Warring States Period was quite an unstable time. Rapid changes would take place all the time, leaving many unsolved problems to the society. Scholars of the day would announce their own opinions depending on the current situation. With many different opinions, a lot of schools emerged.