Selection indicators tell the solver to select certain letters in the fodder (the part of the clue the indicator works on).They can indicate to select almost any letter in a word (if done right) but generally they refer to the first, last or middle letter.
For example, phrases like 'focus of' and 'central character in' indicate to select the middle letters.Phrases like 'beginning of', 'starter in' and 'lead to' indicate to select the first letter.Phrases like 'back of', 'end of' and 'conclusion to' indicate to select the last letter.Below is a link to an extensive list of letter-selection indicators.