They add to the strength of a city. That's the number on top of the city name.
Other info: when you garrison a unit, the city becomes a bit stronger; for example, an archer will give +1 strength to a city, and a melee unit multiple, based on their strength. When you attack with an archer or melee, the unit will not garrison the city. This is especially important when it comes to certain Policies, where unit maintenance, happiness and/or culture is involved.
Anyway, city strength helps to defend, and when you attack with your city, the strength is of course also used to decide what damage is done. City attack is only 1/3 of a normal (melee) attack; so a city with 30 strength will do damage to another unit with only 10. There are 2 policies to bring that back to full, which can be brutal for the attacking enemy.
And to relate to your post: the HP of a city is never changed, it is always 25.
