善 is a good thing to do and 急げ is the imperative of 急ぐ, to hurry, so the proverb is a command: do a good thing at once, before the chance is lost. It is in the first Japanese proverb collection of 1638 with a second half, 悪は延べよ (aku wa nobeyo), put off the bad, and both forms have been current since. From a geisha in 1777 to the uncle in Sōseki's Kokoro of 1914, its speakers use it to hurry a wedding or a bargain.
English equivalents such as strike while the iron is hot are about acting now; the Japanese keeps the goodness of the thing as its object, which makes it a moral maxim and not a counsel of speed.
Philology of 善は急げ