What is the meaning of maskb= (1<<i)-1?

[nitinchauhan0808_be04315530401452 (Nitin Chauhan)]

While creating mask b
maskb=(1<<i)-1;
is used please explain

[talhashamim001 (Talha Shamim)]

@nitinchauhan0808_be04315530401452 it’s equal to (2 power i) -1