Trying to depict with venn diagrams for better understanding..
Left Semi join : A semi join returns values from the left side of the relation that has a match with the right. It is also referred to as a left semi join.
Note : There is another thing called left anti join : An anti join returns values from the left relation that has no match with the right. It is also referred to as a left anti join.
Inner join : It selects rows that have matching values in both relations.