[Web design]How to implement the horizontal par like this?

[Zokusei]

I am a beginner in webpage programming. I saw a interesting menu bar in this webpage. I have tracked the code but I cannot find the related code implementing this.When I scroll it down, the icon/toggle bar always displays at the top. When I scroll it up, it likes a normal webpage(i.e. the whole page moves together).Also, the "Start Free Trial" button appears at different position when it is scrolled down or up.Can anyone tell me the logic or the way to implement the menu bar like this? Is it using JavaScript? JQuery? or other lauguages?It is better if you can tell me the position/location of the related code inside the source code.Thank you very much if you can help my questions.