please help interpret this input using the filter function

Please help interpret this input/output using the filter function.

data = [1:0.2:4]';
windowSize = 5;
filter(ones(1,windowSize)/windowSize,1,data)

ans =
    0.2000
    0.4400
    0.7200
    1.0400
    1.4000
    1.6000
    1.8000
    2.0000
    2.2000
    2.4000
    2.6000
    2.8000
    3.0000
    3.2000
    3.4000
    3.6000

The data is from 1 to 4 in increments of 0.2. The moving window/length is 5, so it is taking the average of every 5 consecutive data. What does the filter do, in this case? Why is the result such? I don't understand what the filter function did here.