This is the front page of today’s edition ofthe Belgian Newspaper, Le Soir. Tenir bon means ‘stand firm’, ‘hold on’ (or, very nearly, ‘keep calm and carry on’).
If you’d like to read a Belgian view on the attacks, here is a link to Le Soir‘s front-page editorial column (which is probably too small to read in the image above), entitled ‘Brussels Attacks: This Is Not The End, It’s The Beginning’.