A slow burn is a romance where the characters take a long time to act on what they feel, and the waiting is the pleasure. The first kiss may arrive two thirds of the way in. In a series it can take a whole book.
What makes it work
Proximity without permission. The two are stuck together by a job, a journey or a shared problem, and something keeps them from doing anything about it: a rule, a loyalty, a misreading of what the other wants. Every scene adds pressure and nothing releases it, which is why readers describe good slow burns in terms of physical discomfort.
How it reads in serialised apps
Serial fiction is built for this and abuses it. When each chapter costs coins, a slow burn is a reason to keep charging, and the delay stops being craft and becomes a meter. The tell is a story where nothing changes between chapters: the same longing, the same interruption, the same near miss. A slow burn should keep tightening, not idle.
Slow burn and heat
Slow burn says nothing about how explicit a book is. A slow burn can end in several graphic chapters, or in one closed door. Readers often pair the tags, asking for a slow burn at four peppers, which means: make me wait, then do not cut away.
Finding it
Slow burn is a tag before it is a genre, so it travels across shelves: fantasy, crime, workplace romance, fan fiction. On platforms with real tagging you can ask for it directly and combine it with a heat level. On app libraries that sort by mood rather than tag, it is usually hiding inside collections with names like "tension" or "forbidden", and the only way to tell is the blurb.
Audio makes the device sharper, because a narrator can hold a pause. It also makes a padded slow burn more obvious: a chapter where nothing moves is harder to skim when you are listening to it.
Related: spicy romance for the heat scale, and the glossary for the rest.