well, neither did I until I got some more insight.
I feel very strongly about this because of how "giving" some of us are (P.S. I know the right word may be generous; but "giving" appeals to me at this time). So let's go on...
Let's start by distinguishing between selffishness and self-lessness and then
zero in on "self-first"...
Selfish means: "looking after own desires: concerned with your own interests, needs, and wishes while ignoring those of others.*"
On the other hand, Selfless means: "thinking of others first: putting other people's needs, interests, or wishes before your own.**"
When you think of these two words as being 2 ends of a spectrum, Self-first sits right in the middle. Any slight move to the right or left can make you self-less, which is preferred since it is a virtue, or selfish - which is a vice and highly discouraged.
To me Self-first means: thinking of yourself so you can think of others...and hopefully take care of their needs, interests or wishes.
I've said it time and again and will continue to say:
You cannot give what you do not have.You need to think of yourself e.g. invest in yourself so you have have something of value to give or share with another person or organisation.