The header and schema are separate things.
Header:
If the csv file have a header (column names in the first row) then set header=true. This will use the first row in the csv file as the dataframe’s column names. Setting header=false (default option) will result in a dataframe with default column names: _c0, _c1, _c2, etc.
Setting this to true or false should be based on your input file.
Schema:
The schema refered to here are the column types. A column can be of type String, Double, Long, etc. Using inferSchema=false (default option) will give a dataframe where all columns are strings (StringType). Depending on what you want to do, strings may not work. For example, if you want to add numbers from different columns, then those columns should be of some numeric type (strings won’t work).
By setting inferSchema=true, Spark will automatically go through the csv file and infer the schema of each column. This requires an extra pass over the file which will result in reading a file with inferSchema set to true being slower. But in return the dataframe will most likely have a correct schema given its input.
As an alternative to reading a csv with inferSchema you can provide the schema while reading. This have the advantage of being faster than inferring the schema while giving a dataframe with the correct column types. In addition, for csv files without a header row, column names can be given automatically. To provde schema see e.g.: Provide schema while reading csv file as a dataframe