Strolls are a type of line-dancing using repeated sequences of steps so that you get to practice the steps as a group. Some are simple and other are complex and all improve your dancing by developing your range of steps, balance, style, confidence, etc.
A Stroll is a group dance rather than a partnered dance. Many are commonly danced to medium-slow tempos and others are more lively. They allow you to take a break from partner dancing at social events and offer the opportunity to join a group on the dance-floor - even if some in the group are dancing a few variations to the basic version (as in the first video).
Once you learn few steps you will be ready to add a few more. Once you have learned a few Strolls you will find that it all gets easier because you get to use some steps that you already know in a new pattern.
The videos shows a very popular simple strolls - look out for the dancers who are adding a few minor variations.