Indie is basically independent developer, it isn't a genre. What has happened though is that for the sake of simplicity, they have all been lumped together.
The quality and budget can vary between high or low. But those games can range between larger budget and quality titles like Control, Kena, or Returnal to UntitledGoose, UnfinishedSwan, or Bloodstained. House Marque is independent, Insomniac was independent until Sony acquired them, Kojima productions is indie, Remedy is a indie, and the remake gods, Bluepoint.
Most associate indies with A games, but forget that publishers can have smaller portions of owned development teams make them as well. All the Indie moniker is a title that the studio isn't owned by a publisher. A lot of Sony's 2nd party games come from famed indie studios. Just as there are low quality AAA, there are low quality indies. The primary reason a majority of indie titles are often A is because that is mainly the cost that studio can afford or perhaps is the highest bar they are striving for.