I want to access my /opt/ folder I have found following commands for giving access permission.

But not sure what is the purpose of this commands which one is the better to use it to maintain security and access permission both.

Please explain me the purpose or what this different numbers means in security permission ?

here are they , sudo chmod 755 -R /opt/

sudo chmod 755 /opt/

sudo chmod 775 /opt/

sudo chmod 777 /opt/

I dint know these commands so what I use to do previously was "gksudo nautilius" then rightclick-> change the owner from root to current usergroup

But now as I have found this commands I would like to know Which one should i use ? and what they do ?