Lock Screen : it make screen to turn off and lock. it allow programs to run in background so all your work will run in background when you lock the screen. example if any command was executing and you lock the screen. the execution of command will run in background. to unlock screen you will have to enter your password. to lock the screen execute
xdg-screensaver lock command. or use shortcut window + l
Using Desktop Menu : click on menu button near battery icon(on top of the right hand side) > select middle option ( lock icon see in example 1)
Restart And Power Off : following are ways to restart or poweroff ubuntu.
Using Desktop Menu : click on menu button near battery icon(on top of the right hand side) > select last option (background highlighted see in example 1) from left > select your option(Restart, Power Off, Cancel)
restart and poweroff menu restart and poweroff menu
using terminal :
reboot: this command will reboot ubuntu.
poweroff: this command will shutdown or poweroff ubuntu.