To use the Point Cluster tool, you must first be in the Layer Editor and head to the Styles Tab. If you scroll down and open up the drop-down for Point Clustering, it gives you the option to do Grouping, create a Heat map, create a Density point map, or turn it off. Choosing Grouping combines points into groups with the number of points displayed, and gives you the choice to turn on Proportional Sizing and adjust sizing to your liking. If you choose Heat, you can create a heat map based on the attribute of your choice. If you choose Point Density, it creates a heat map based on the number of points in the area rather than on an attribute associated with the layer. From there you can choose to adjust the Weight and Intensity, as well as Max Zoom, Radius at Zoom, and choose to adjust the Color Ramp manually (setting your own range) or automatically (choosing a Color Ramp).