Morphological operations are normally used to "tidy up" particles to remove any unwanted small particles, or join together particles that should not be isolated. For a detailed explanation of morphological operators, please see this link.
If you set more than 1 morphological operation, these will be applied sequentially. Each morphological operation has its own panel:
For each morphological operation, you can choose:
Having set the morphological operations (if any), you can now decide on the connection strategy to be applied.