Health visitors used to all be nurses or midwives who'd done extra training, but now it's possible to train directly to be a health visitor. Their role is to support families with children under the age of 5. They offer support and advice on infant feeding, weaning, potty training, sleep, behavioural issues and mum's mental health. They also do routine child health surveillance visits. Unfortunately, as with all these things, they are very variable in quality, with many seeming still to give very out dated and UN-evidence based advice.
DD has seen a HV 3 times in total. Once at a few weeks old, then we missed the 4 month visit (they'd left a message on the land line we never used!), then at 1 and again at 2.5. She got weighed at a HV clinic when she was little, too, but they only weighed her, never did anything else. People who are struggling more would have a lot more input, and you can always phone for advice if needed.