Hello.
I'm fairly new to bitcoin mining and recently learned that in the near feature, the block reward will halve from 25 BTC to 12.5 BTC.
What exactly does this mean? Will this mean that miners will make less BTC since pools have less to split among the workers?
Short form answer, that is correct.
Long term answer, transaction fees add a little to the reward for solving each block - but at this time most blocks have quite a bit less than 1BTC of transaction fees included so they can pretty much be ignored.
VERY long term answer, eventually the transaction fees will be most of the reward for mining a block - but that's years out, possibly a decade or more out.
Between the difficulty increases and the block halfing, an S5 will probably become unprofitable to operate after the halfing - even if you have VERY VERY cheap (3c/KWH or less) electric it will most likely cost more to operate than you'll get in BTC rewards at that point. The only way it will continue to be profitable is if Bitcoin prices stay bumped up quite a bit or increase a fair bit more between now and then, in which case it MIGHT be minimally profitable for a short while after the halfing - but I'd bet AGAINST that.