Final answer:
The line of code in question is likely from a pandas library method involving an 'outer' join between two dataframes, ydf and zdf, in a programming environment like Python.
Step-by-step explanation:
The snippet (ydf, zdf, how='outer') relates to the subject of Computers and Technology, specifically within the context of data analysis in programming environments such as Python. This format is indicative of a method call, likely from the pandas library, which is utilized for data manipulation and analysis. The mention of 'outer' suggests that the method in question might be related to merging or joining two dataframes, ydf and zdf, where 'outer' specifies the type of join operation.
In an 'outer join', all rows from both dataframes are kept, with missing values filled in where they do not overlap. This differs from an 'inner join', where only rows with matching keys in both dataframes are included. 'Outer' joins are instrumental when one needs to preserve all data, accepting that some data will have NaN (Not a Number) or null values for the mismatching rows.