Means if you send the bot 1 Steem, you might get between 0.90 Steem (-10% ROI) to 1.15 Steem (15% ROI) back as SP and Steem for author reward.
That just simply means: the minimum ROI (return of investment) you can get is -10% and the maximum ROI you can get is 15%.
Most bidbots usually calculate ROI in terms of USD. If you send in $1 USD worth of Steem, you are guaranteed that at least 90% will return back to you (-10% ROI) or if you are lucky and there are few bidders then you can at most only make 15% profit out of it.
Many bidbots were somewhat forced to add this limit because there were users who will send in large bids at the last minute.
Consider a scenario wherein the full upvote worth of a bidbot is at $50. If you bid in $5 and then someone accidentally bids $60 at the last minute, then it will surely be a loss to both of you as the bidbot's full upvote is only $50.
To solve this, bidbots put in some limitations in which it will reject bids that will put the ROI below the threshold they set (-10%/+15% in your example).
That means don't invest in bidbots, because you may lose 10% of your investment, and the maximum ROI (return on Investment) is 15% profit.
There are free upvote bots, for example Busy, The Minnow Support Project, Swagger, SM-Voter (only for Steem Monsters posts), and OnePageX. You can earn money by using them. Especially in the long run. Maybe there are more free upvote bots, but these are what I currently know of.
That means that whatever you invest into a bid-bot you would earn a minimum ROI of -10% which means that you could possibly even loose ten percentage of your investment,,that is not a very good idea but at least your contents could get more visibility....
The 15% ROI is the maximum you will get on your investment which means that you can gain 15 percentage profit on your investment....
The fact is that many people who use bid bots are not using it as a way to get returns on investment,they are actually using it to make their contents on steem platforms to become more visible and being hopeful that the more visibility their contents get then the more upvotes they will get..