Field length and average field lengths BM25

I dont quite understand the forumula of the BM25 explained here:

What is menat by the field length?

Field length is a number of terms in the current field of the current document.

For example:
if a doc1 has a field field1 with a value "foo foo bar", its field length will be 3.
if a doc2 has a field field1 with a value "foo foo foo bar bar", its field length will be 5.

If these are the only two documents in the index, the avgFieldLen of field field1 will be = ( 3+5)/2 = 4

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Thank you very much!