问在Trie结构中，"charAt(i) - 'a'“是什么意思？

EN

// Returns true if key presents in trie, else false 
static boolean search(String key) 
{ 
    int level; 
    int length = key.length(); 
    int index; 
    TrieNode pCrawl = root; 

    for (level = 0; level < length; level++) 
    { 
        index = key.charAt(level) - 'a'; 

        if (pCrawl.children[index] == null) 
            return false; 
        pCrawl = pCrawl.children[index]; 
    } 
    return (pCrawl != null && pCrawl.isEndOfWord); 
}