They're systems for parsing logs and other data, it normalises and indexes this so that it can quickly be search and aggregated, with the results visualised or alerts set up for them. For example, it could suck in all the logs from your Firewalls, you can then aggregate this data to see what the top 10 threats are, or search where those threats came from and display them on a map.
Splunk has enjoyed a monopoly on this for the past several years and their pricing model reflects this, charging by the GB of data is processes. For example, if your collecting 10GB of data per-day, cost if about £7500 per annum - Lets face it, who knows how much log data their servers and firewalls are going to generate on a day-2-day basis.
Personally I hope Rocana wins this one, as I think the per-GB model is a relic.. its actually one of the reason ELK is being so successful... Also for anybody looking for another alternative, I highly recommend checking out Logscape, licensing is based upon number of indexers, and since 1 indexer (which costs less that a 10GB Splunk licence) can parse 75-100GB per day, its not bad value for money.