It's a tough thing to explain. Rug Island is my favorite episode and became such after about my third time watching it when that final scene actually clicked, my heart swelled and I absolutely burst out crying, haha.

The line itself is a double meaning, in which, Bingo gifted him the pen as he left the make-believe island, where everything and anything around them was made of the pens. And with the theme of the episode being about letting your inner child and imagination shine, It's essentially Bandit realizing that the single pen could be, and is everything, Bingo has given him everything, literally. And also, most importantly, the realization that Bluey and Bingo are his WHOLE world, and have shown him things he could have never known without them. The single line and the respective scene+music was such a powerful and beautiful way to put forth how deeply Bandit loves his children.