I would summarize the meaning of "secular humanism" mostly as a deceptive cudgel used by conservatives to lump all those who wish to see politics and governance be separate from religion, as in the wording of the 1st amendment; "Congress shall make no law respecting an establishment of religion". This is a vague wording, "an establishment of religion" could mean a church or synagogue, it could also mean the act of establishing religion into law...argument abounds.

No doubt there is an actual organized group or school of thought called secular humanism, but this is not what conservatives mean when they take broad swipes at a supposed and possibly non-existent movement to remove religion from the daily life and values of the nation, and this is the use of the term most Americans who actually are familiar with the term think of.