If something is semiflexible, it can bend a little bit but not too much.
Semiflexible things are not too stiff and not too loose. They can move a little bit but not too much.
Examples:
The plastic ruler is semiflexible.
The tree branch is semiflexible.
The semiflexible straw can bend a little bit.
The semiflexible toy can bend but not break.
My superhero cape is semiflexible, so I can fly easily.
The semiflexible bridge can move a little but not too much.