What is MUTUAL Fund & is it safe for general ones to invest ?

I’m a guy of 26 years old and doing job in BPO sector
I invest my all salary in bank because I don’t know what is mutual fund
Can anyone pls tell me in a easier way (bcos whn my frnd try to xpln me I didn’t get it at all but I act as if I got it ) so pls xpln dat lucidly
as one of my friend gets profit an another one looses
so what should I do ?