So if I send 100k to SN, I have new inbound liquidity?

Fabs

To send money, you must have outbound. i.e. you have put money into a channel with someone else.

To receive money, you must have inbound. i.e. someone has put money into a channel with you.

lightning

Who can give me an ELI5 on Inbound liquidity and it's increase / decrease?!

There is no credit in Bitcoin land. 

To send money, you must have outbound. i.e. you have put money into a channel with someone else. 

To receive money, you must have inbound. i.e. someone has put money into a channel with you. 

Simple as. 