When you turn on dynamic labeling, the Standard Label Engine fits as many labels as possible, without overlap, within the available space on the map. Label placement works with the space available for a feature. If a feature is only partially visible at the edge of your map, the Standard Label Engine tries to place its label using the visible portion of the feature. As you pan and zoom, the Standard Label Engine automatically adjusts labels to fit the available space and the available portion of the feature. For many maps, labeling using the Standard Label Engine is adequate. If you need more control over which features are labeled and where labels are placed, use the priority, weight, overlap, and placement options. Additionally, if you are labeling line or multipart polygon features, you can use duplicate labels. All of these properties work on a label class level.
If you need further control while placing labels, change to the Maplex Label Engine.
Learn more about labeling with the Maplex Label Engine
As you zoom in and out on your map, the size of the labels does not change. If you want label text size to scale with the map, set a reference scale.
Position labels
The Position tab 
in the Label Class pane provides label placement options to control the positioning of labels with respect to features. For example, you can specify that your city labels always be placed above and to the right of the cities.
The Standard Label Engine has different label placement options that depend on the feature type. Point labels, line labels, and polygon labels are all placed with different placement options.
Resolve conflict between labels
The Conflict Resolution tab 
in the Label Class pane allows you to set conflict detection rules to determine which labels are placed when multiple labels are placed at the same location. These conflicts can be resolved using weights, buffers, and label overlap.
Weight labels and features
Label weights and feature weights are used to assign relative importance to labels and features. This weight is to be used only when there is a conflict, that is, an overlap between a label and a feature. The general rule is that a feature cannot be overlapped by a label with an equal or lesser weight.
Buffer labels
The Label buffer ratio option makes the label size larger than the actual text, preventing labels from being placed close to each other.
Place overlapping labels
Specifying this option forces placement of a label for each feature in this label class regardless of whether they overlap each other. By default, ArcGIS Pro automatically resolves conflicts between labels and does not allow them to overlap. You can change this behavior using the Place overlapping labels option for a label class.
Prioritize labels
The Label Priority Ranking dialog box controls the order that labels are placed on the map. Labels with a higher priority are generally placed before labels with a lower priority. Also, lower-priority labels that conflict with higher-priority labels can be placed at alternative positions or removed from the map.