It is recommended to seal one's BOS so all symbols recorded do not manifest by accident.
Sealing a book of shadows makes it so all runes and sigils recorded do not hold meaning, come into effect, or otherwise manifest in the real world by accident; For example, if you write down a sigil that symbolizes bad luck in a sealed book of shadows, it is still written on the paper, but it won't bring bad luck to the book of shadows or its surroundings, whereas if the same sigil were to be written anywhere else, it would bring bad luck to that surface, area, or to the target of any spells done near it, depending on the caster's intentions.
Sealing is generally done with the following prayer, or a version of it-
"I call upon (deity/spirit/element)
so mote it be
to seal this book's pages
by the power of three times three
from all symbolic manifestation
blessed be thee"
3 or 9 times while holding the Book of Shadows. (3 is believed to be a special number, so of course, the product of 3 multiplied by itself is also significant.) If you'd like, you can also imagine the energy from inside and around you moving to Book of Shadows and enveloping it in a protective layer of bright light.