I like this saying.
You can interpret it a number of ways, but the way I like to think of it is this:
Have a strong opinion- whatever it takes to get there; research, discussion, learning etc.
Don't just have an opinion, make it one worth holding, make it strong.
But hold it loosely.
What does this mean?
It means, be ready to change your opinion if you get better, newer, more accurate, more useful information.
To have a strong opinion, is not the end goal, it's the starting position.
To hold it loosely, brings you into the game- where you, and others, put yourselves in a position to advance, progress, develop and evolve those positions and knowledge, not to hoard, stand still and be rooted to the spot.
But be ready to change, advance, progress towards your north star.
So, form strong opinions, and hold them loosely so you may grow, and move, and see the world.