Welcome to OCDO.
"Retention" generally means that something keeps the handgun from falling out of the holster due to the nere application of gravity. Could be a tensioning device, such as what's used with the holster you pictured, or a strap (thumb-break or wrap-around-the-hammer/rear-of-the-handgun), or bells ans whistles that raise the holster to Level II or Level III or Defcon 7. All those bells and whistles are just stuff to make it harder for the gun to come out of the holster unintentionally. Most folks justify the additional levels because they want to prevent a gun grab - the one that has happened so infrequently as to be "statistically insignificant" to everybody but the person whose handgun was in fact pulled out of the holster by a BG who then used it against the OCer.
Cops and "operators" like Level III/Defcon 7 holsters. Oulde Pharts like me use either an open-top or thumb-break strap depending on how tight the leather holds the handgun. The holster you pictured is a step up from my open-top and equivalent to the retention offered by a thumb-break strap.
I've always heard that your holster should hold your handgun in while doing most of the physical activities you are likely to perform. Since I no longer hang upside down on the monkeybars I am not too worried about my handgun falling out because I'm upside down. Same for worrying about it coming loose while I'm running (you've got to be kidding, right?) or rolling on the ground (only from laughing these days). I need something that keeps my handgun handy while driving and while sitting - usually in front of a computer. YMMV.