The recipe is algorithm used by jsreport to convert the output of templating engines into desired format. Every report template needs to specify exactly one recipe from many of them provided by jsreport. Specifying the phantom-pdf recipe will for example create pdf report using html to pdf conversion. Using html-to-xlsx on the other hand produces excel file.
jsreport usually supports various recipes for particular output type. This is because every recipe has its pros and cons. It is recommended to compare multiple recipes and decide which fits the most for the particular case. You can find useful also pdf recipes comparison in the dedicated article here.