From your screenshot, I presume your question refers to Service Area analysis layers and not the Generate Service Areas ready-to-use tool, which is a specific thing. Your screenshot appears to show part of the Travel Mode tab in the Service Area layer properties dialog.
A Travel Mode is a collection of settings that model a particular mode of travel or vehicle type. The travel mode includes things like the cost attribute to be used for the analysis (the thing being optimized) and restrictions, u-turn settings, etc.
The Cost Attribute (also called Impedance Attribute) is the thing being optimized in your analysis, such as travel time or distance. It appears that your network dataset has a cost attribute called TravelTime. The Service Area will calculate the TravelTime for each road segment starting from the facility and travel outward until it hits the limit specified.
Attribute Parameters are properties of cost attributes that control or modify their behavior. For example, if you had a cost attribute that modeled walk time, that attribute might have a parameter for walk speed. The cost would be calculated by dividing the length of the road by the walk speed parameter value. You could change the parameter value at solve time to increase or decrease the walk speed, and that would change the results of the analysis.
Here is some documentation about parameters: Use parameters with network attributes—ArcGIS Pro | Documentation
The Service Area layer's Travel Mode properties let you examine and update the various travel mode properties. If you click on any of the rows in that Cost Parameters table, it should expand and show you a detailed view of the parameters that apply to that cost attribute and their values for the particular travel mode your layer is using.
From your screenshot, it appears that your TravelTime cost attribute has some parameter whose value is Null. If you click the row in the table, it should expand and show you what the parameter is called. I can't really guess from the information you've provided, and it's not clear whether you would need to enter a value there to produce good results. Hopefully whoever created the network dataset configured the cost attribute and associated parameters correctly such that Null is a valid value.