Meaning of ChillSuru(チルする)

ChillSuru(チルする) means relax and it’s from chill out in English.
This is a slang among young.
Suru(する) means doing.

Example Conversation

A: What did you do yesterday?
B: Chilltteta(チルってた) at café.

A: 昨日何してたの？
B: カフェでチルってた。

*tteta(ってた) is past tense of do