Nostalgia is a feeling one gets when talking about good things in the past, specifically: a sentimental longing or wistful affection for the past, typically for a period or place with happy personal associations.
It is used very commonly. It is a feeling most if not all of us experience, and as such, we commonly refer to it, like sadness, happiness, loneliness and other common feelings. One can even feel nostalgia for an experience we never had, for example, when looking at a depiction of a happy family. Norman Rockwell was known for making people feel nostalgic about a past they never experienced.
I have experienced nostalgia many times. Usually I do so when thinking of simpler times, or times when I was particurlarly happy. For example, when I think about the farm I had 20 years ago, and how much fun the animals were, how much joy they brought to the hearts of my children and myself, I feel nostalgia for those days, and for my animals.