Ah, I see. "V1", "V2", etc are the column names of the data frame. A data frame has to have column names, and since your data did not contain any, the import function created default ones for you. Were you expecting that the dates would be the column names?
In R data frames and the many functions that work with them, columns are generally assumed to represent variables and rows are generally assumed to represent observations. For your data, you have two variables — date and rainfall measurement — and 504 observations. You can force R to create a data frame with one column for each observation and only one row, but this is not going to be easy to work with down the line.
Given your data, perhaps you wanted to create a time series object, not a data frame? You can read more about them here: https://www.rdocumentation.org/packages/stats/versions/3.5.1/topics/ts Typically, data import functions read data in to a data frame, which you can then convert into a ts object. ts objects are somewhat limited — if your data don't fit ts()'s assumptions, you'll probably want to look at the zoo or xts packages.
Again, for more specific advice or help, you will need to create a reproducible example or post your data import code and a sample of your data.