You may at some point run into geometry like the two disjointed bodies shown below:
You might be tempted to quickly extend the solids using a boundary or loft feature between the two, but this usually leads to a fairly poor transition between the bodies. The best way to approach this issue is to actually remove out material and then connect the bodies, using a boundary feature, which very similar to the boundary surface tool.
Check our the full SolidWorks tutorial below: