Openly and publicly be consistent in demonstrating what one personally wants.
Breaking the words down… “Stand” gives us the consistent part, as opposed to some kind of moving around, such as wavering, running away, pushing, sidling, etc.
The “up” gives us the public part, as opposed to laying low/down.
The “for” part means the positive, wanting stuff, as opposed to being against stuff.
And the “oneself” part is doing it openly (full disclosure of one’s true self), as opposed to standing up for “someone, somewhere”.
Clearly, there is no pushing other people around, being negative, hiding, or saying one thing to one person and another to another person involved. Sometimes these other things are valuable, but they are not “standing up for oneself” at least not in any kind of accurate or literal sense.
This is something that most folks aren’t ever taught! Imagine if kids learned this in kindergarten, so that they had a logical and literal understanding of how they can, indeed, stand up for themselves when confronted by bullies?