What does this "ICAgICAg…" mean in public key certificates and messages?

ICAg represents three spaces when text is base64-encoded.

For example:

echo -n '      hello world' | openssl base64

produces:

ICAgICAgaGVsbG8gd29ybGQ=

The accepted answer doesn't show how to get the answer (it shows, how to verify it). Use

echo 'ICAgICAgICAgICAgICAg' | base64 -d

(producing a bunch of spaces) or

echo 'ICAgICAgICAgICAgICAg' | base64 -d | hexdump -C

producing

00000000  20 20 20 20 20 20 20 20  20 20 20 20 20 20 20     |               |
0000000f

to see what's inside.