A single bet is a bet on one match only. Single bets are very popular in sports betting cause the propability to win is high.
Many sports betting experts try to find the strongest match of the day and bet high amounts on this match. On this way the propability of winning is high and you can win the maximum. To be a winner on long term your selected match be the most times successful.
Other sports betting experts try to choose many strong matches in one day and bet a fixed amount on all of them.
This has the advantage that you can bet incorrect on some matches but at the end of the day you can still have a positive profit. But therefor you should bet correct more matches than incorrect. For example you bet a fixed amount X on 7 matches with odds about 2,0. In this case you should succeed on 4 matches to have a profit at the end of the day.
Finally, we can keep that single bets can be profitable if someone has s good sense for sports betting. And it is not neccessary to bet correct on all matches. It is ok to select more correct matches as losing matches.