I'm trying to understand how commodities futures are priced. I was looking at a chart of ZSF18, and the price is 990-4s. What does that mean?
Some futures markets are priced using a fractional pricing rather than decimal pricing.
The example you've mentioned, Soybeans, is priced in cents per bushel, and the "tick size" (the minimum amount the price can move) is 2/8ths of a cent. So, 990-4 means 990 and 4/8ths of a cent.
Also, it's not just some commodities priced this way - some interest rate futures are also priced this way.
There are more complex versions of fractional pricing too. Take 5-year Treasury Notes that trade on CME. The minimum tick size is quarter-32nds (i.e. quarter of a 32nd of a point). In decimal terms that is 0.0078125. For example, the notation for 5-Year Treasury notes trading at 109 and 18/32nds and 3 quarters 32nds is 109-187. The "7" in this example is the convention for a 3/4 value - the trailing "5" is left off. The next tick up would be 109-190 and the next tick down would be 109-185.
Note that other sources sometimes use either a carat character (^) or a space between the point and the next fractional part, so you might see it defined as 109^187 instead.