A “banker” can apply to any betting odds in any sport in theory, but in the social media circles that we live and work in it generally refers to betting on big favourites, that would by default have a high percentage chance of being successful.
It's probably important to understand where this term originally came from. The word banker refers to a feature used as part of a system bet when betting on most commonly football and horse racing. A basic example is as follows:
You select four separate bets. Rather than placing an accumulator and needing all four of them to win, you can use a “banker” bet. You select your “banker” - your strongest bet - and your bookmaker compiles a combination of bets with all of your selections. Each combination bet includes you banker, and with four selections could look like this:
Doubles (3 bets)
Banker + A
Banker + B
Banker + C
Trebles (3 bets)
Banker + A + B
Banker + A + C
Banker + B + C
The stake is split into three bets if you bet the doubles (see above), and the same would happen with trebles in this example. The logic is pretty simple. If your banker loses, all bets have lost. If the banker wins, then if you've backed doubles, you know each winning bet thereafter will result in a winning double. If you've backed trebles, then for every ‘pair' of correct bets (A+B, B+C, B+C) you'll have a winning treble.
This method can be pretty popular, and even profitable if done well. It also has it's benefits over an accumulator, as it means you can still get return should one of your selections fall short.
Betting 'bankers', the good points
The term as it's commonly used - to suggest a bet that's all but certain to win - need to be used carefully, but can accurately describe a particular selection that offer exceptional value. If you know you're following a reliable and professional tipster, then you know such terms won't be used lightly. The term banker or “cert” can never be taken literally, but they are words used to highlight a bet that a tipster feels particularly strong about.
If you're looking to be select and perhaps happen to follow a service where the volume is slightly too high for your liking - but you know you trust your tipster - then there's no harm in keeping your betting selective and keeping an eye out for these buzz words.
And now, the bad points
The term banker, which we've already discussed, can be misleading. It's impossible to find something that is a certain bet. It's commonly used by unverified tipsters or tipsters who exist purely on social media to drum up likes and interactions - be wary when you see it, unless you already know the tipster, and know their credentials.
That said, can a low priced banker offer value? Statistically there may be value on occasions for low priced bets, but typically, the lower the price, the harder it is to be able to find and determine value.
The danger with this term comes in the way the term is used so freely on social media, which often encourages people to bet larger amounts of money than they usually would and begin taking unnecessary risks.
The betting banker, a summary
Remember that statistically there is no such thing as a certainty in sports betting, and when you hear the word banker in the future, you shouldn't rush to bet more than you usually bet and you should never bet more than you can afford to lose, as there's always a chance that it'll lose. If helps to know the credentials of the tipster/sports bettor using the term before you give it any attention.