goto finspreads.com or deal4free.com etc. They all have examples of how spreadbetting works.
Then seach this BB for plenty of in depth discussions about every aspect of spreadbetting.
In a nutshell, you "bet" something ( a stock or an index) will go up or down. You bet so much per penny rise/fall, or , in the case of an index, per point rise or fall.
You think the FTSE will go up from 4000 so you bet £1 per point rise. It goes up to 4050 and you close the bet. You make £50. Simple in theory, complicated and hard in practice. You need to study the whole issue.