By the time you find the meaning, you have memorised the word. When you eventually find the meaning, you have memorised the word. 'By the time...' suggests it takes some time to do something, as in this example, it take a while to find a word in the dictionary. Another example: By the time I get home it will be dark. When I (eventually) get home it will be dark.

In your example, it means that it will take some time to get the meaning of the word, and when you do get the meaning, you will have memorized the word. The feeling is that it will take time to get the meaning of the word.

It is a signal word of future perfect. http://www.englishpage.com/verbpage/futureperfect.html