A sequence refers to a series of images captured and then uploaded to Mapillary based on the time and place of capture. When you explore images on Mapillary, we use sequences to structure the viewing experience. Each image, in addition to a unique image key, has a sequence key that links it together with other images in the same sequence.
We prefer sequences instead of single images because we are building a 3D reconstruction of the world with our computer vision technology, and to that end, it is important to have lots of overlapping images of the same place. This is exactly what you achieve with capturing images in sequences.
Regardless of the images being grouped into sequences, they are still all interconnected on Mapillary. You are able to navigate around both within and between sequences by using the sequence arrows at the top or the spatial arrows at the bottom, respectively.