touchtouch sets the modification time and access time of files use -m/-a flags to set only one of the time individually. That is, it update the fields struct timespec st_atim and struct timespec st_mtim in struct stat of a file.
By default, If any file (in the given file list) does not exist, it is created with default permissions (ironically this is the most common usage of touch). Use -c to disable the creation if it does not exist.
To specify the time manually instead of the current time, use -t to give the absoluate time or -A to give the relatvie time to the original one.