What does the author really mean when he says "When all at once I saw a crowd, A host, of golden daffodils"?

The author isn't referring to a person by crowd and host, but is instead referring to an abundance of the daffodils.

Explanation:

in other words

to put it differently

for one thing

as an illustration

in this case

for this reason

to put it another way

that is to say

with attention to

by all means

important to realize

another key point

first thing to remember

most compelling evidence

must be remembered

point often overlooked

to point out

on the positive side

on the negative side

with this in mind

notably

including

like

to be sure

namely

chiefly

truly

indeed

certainly

surely

markedly

such as

especially

explicitly

specifically

expressly

surprisingly

frequently

significantly

particularly

in fact

in general

in particular

in detail

for example

for instance

to demonstrate

to emphasize

to repeat

to clarify

to explain

to enumerate