There is a bottle of glue in the lower left corner and there are letters that give the illusion of being cut from magazine text and glued to the billboard. The letters appear to form one word adhere; however, the letters a and d are in the same font which is a different font from the letters h, e, r, and e that are also in the same font. Does that make sense? Like this: adhere.
So you have the word adhere with the syllables ad and here, which are also the free morphemes (or words) ad and here. Semantically this delivers two messages:
1. The viewer of the billboard, as a potential advertiser, could put their ad on that billboard
2. If the viewer of the billboard puts their ad on that billboard, their message will adhere with future billboard viewers, in other words, the message will stick to the future billboard viewers memory (which is key in advertising).