Ditto what the others have said. Once you get your red book you'll see they have a section to record babies weight and length etc and any notes from visits they make. They will weigh baby regulary for a number of weeks and then you can have weight done at your loca HV clinic usually run once a week someplace. She can explain all.
Its not like they are there for a witch hunt or anything but to make sure things are going ok and to offer advice, support and information. They can help tell you about local groups etc also.
If you have any concerns about breast/bottle feeding, baby, yourself then you can talk to her and hopefully she'll be able to help. They are there to help.