[H] Android what am I not getting

[_ Winterlight]

I have a Android phone. I like to listen to news or Youtube videos while I work 
around the house. I wear headphones and put the phone in my pocket. Often when 
I move the pocket makes contact with the screen the video pauses, or turns off, 
or I load six different apps!  This can't be designed like this but I have 
googled and tried to figure out how I lock the screen without turning off what 
I am trying to listen to or work with. What am I missing? What am I doing 
wrong?  Thanks
w