What it means
I'm just looking. The repeated 看看 softens it — a single 看 would sound blunt.
When to use it
The polite escape from an eager shop assistant. Chinese shops often have staff who follow you, and this is the standard reply.
Examples
xiè谢xie谢，wǒ我zhǐ只shì是kàn看kàn看。
Thanks, I'm just looking.
wǒ我zhǐ只shì是kàn看kàn看，xū需yào要zài再jiào叫nǐ你。
I'm just browsing, I'll call you if I need help.
tā她zhǐ只shì是kàn看kàn看，méi没mǎi买shén什me么。
She just looked around and didn't buy anything.
The full set of survival phrases for greetings, dining, shopping, and getting around, each with audio.