fill in

1.  Complete something, especially by supplying more information or detail. For example, Be sure to fill in your salary history. It is also put as fill in the blanks, as in We'll rely on Mary to fill in the blanks. Yet another related usage is fill someone in, as in I couldn't attend, so will you fill me in? The first term dates from the mid-1800s; the others from the first half of the 1900s. Also see fill out.
2.  Also, fill in for. Take someone's place, substitute for. For example, The understudy had to fill in at the last minute, or I can't come but my wife will fill in for me. Also see fill someone's shoes.

Definition: answer in writing
Antonyms: leave blank

The verb has 4 meanings:

Meaning #1: supply with information on a specific topic

Meaning #2: represent the effect of shade or shadow on
  Synonym: shade

Meaning #3: be a substitute
  Synonyms: substitute, sub, stand in

Meaning #4: write all the required information onto a form
  Synonyms: complete, fill out, make out