Learn French: Ne traversez jamais ce pont !

Because this is a command (the imperative mood). Just like in English where we say "Cross!" instead of "You cross!", in French you drop the subject pronoun. So, vous traversez (you cross) becomes simply traversez (cross!).

How does the negative ne ... jamais work with a command?

In a command, the negative words wrap around the verb exactly like they do in a normal sentence. Instead of the standard ne ... pas (do not), this sentence uses ne ... jamais (never). So, ne goes before the verb and jamais goes right after it.

Who is being told not to cross the bridge?

Because the verb ends in -ez, we know it is the vous form. This means the speaker is talking to either a group of people (plural you), or to one person they are being formal with.

Why is "this" translated as ce here?

Ce is a pointing word (a demonstrative adjective) meaning "this" or "that". It is used here because the word for bridge, pont, is a masculine noun. If the noun were feminine, you would use cette, and if it were plural, you would use ces.