A consecutive-day pass provides unlimited rail travel for the duration of the pass, once it has been validated. For example, if you buy a month long pass you can travel at any time during that month.
Flexible travel days give the pass-holder a 2 month period from the validation date to utilize the number of days they have purchased for their pass. If you buy a 2-month flexipass with 10 days of travel, you can use the ten days at any point during the 60-day period.
Both passes have their benefits. The consecutive day pass is great if you are roaming and don't know exactly where you are going or how long it will take you. It tends to be more expensive. The flexipass only allows limited days of travel so more planning needs to go into your itinerary. It is great for travelers on a budget and costs less than a consecutive day pass.