Si is one's awareness of long-term memory. It is what gives somebody their sense of duty and need for honour (although the latter is usually Si with Fe), tradition and stability.

If Introverted Sensing were an element, it would be Earth.

They are past focused, duty-based, focused on safety, comfort-based. Since they are focused on the past, they are likely to remember things that happened long ago. They store things in their memory and so may have a longer reaction time, but that time will be used to process the information and store it. 

Introverted Sensors are very loyal, and you shouldn't push a loyal person to the point where they start not giving a damn. They want to be as comfortable as possible and if you make them uncomfortable, they will backstab you. Because everything you do, they will remember. (Of course, only make them comfortable when it's realistic or responsible to do so.) Introverted Sensors are also very patient. Which makes them enduring, especially ISFJs.

Si is all about telling stories, especially of their past. Types with Si child -- INTPs and INFPs -- do it the most. 

They love recieving experience -- which comes from Se users. They themselves may alos give experience to others, but it will be more like options; Do you need this?

Si paired with Fe are very traditional. They honour tradition very much. Si Child functions (INTPs & INFPs) would appreciate it and Si Inferior (ENTPs & ENFPs) would sometimes just not be bothered. Si Hero (ISTJs & ISFJs) would demand it, and Si Parent (ESFJs & ESTJs) would expect it. 

Si is paired with Ne, so they use their past experiences to see patterns and prognosticate into the future and provide options.