Fetching details from the dictionary...
shyness; fear of strangers; stranger anxiety
赤ちゃんが人見知りする。
The baby is shy with strangers.
息子は人見知りが激しくて困る。
My son is extremely shy, which is a bit of a problem.
彼は人見知りなので、歓迎会を欠席した。
Because he is shy, he skipped the welcome party.