PP's above have explained it very well.
Parents usually come to me saying things like:
They're chewing their fists
They're watching me eat
They're wanting more breast/formula milk
They're waking in the night for milk when they used to sleep through.
These are all normal baby behaviours, not signs that your baby is ready for solid food. As your baby grows they will need more milk, because they're getting bigger and more active and need the extra calories.
Breast milk or first infant formula provides everything your baby needs until they reach 6 months of age. After this they need extra nutrition from solid foods.
Signs your baby is ready:
stay in a sitting position, holding their head steady
coordinate their eyes, hands and mouth
so they can look at their food, pick it up
and put it in their mouth
swallow food (rather than spit it back
out)
There is no reason to wean earlier than 6 months unless you have been advised to by your child's paediatrician, GP or HV. It's very rare that professionals will advise that though. In the last 2 years I can only think god 3 babies out of a caseload of about 6000 that have been given the advice to wean at 5 months and those babies had very specific health needs.