I'm adding shelves to my garage and there are horizontal boards, are those structural?

Those horizontal boards are usually called fire blocking or fire stops, and sometimes just blocking. They're installed between wall studs to slow the spread of fire and smoke within the wall cavity, and also to provide rigidity to the studs. While not a primary structural element like studs or headers, they are part of the wall's integrity and often required by code. Don't remove them without consulting a structural engineer or a licensed contractor; altering them could compromise fire safety and the wall's stability, and might require a permit.