The English term "wagern" means something like "to bet". Sometimes players read about the "wagering conditions" in online casinos. This refers to the wagering conditions that must be met before a previously granted bonus can be released in real money and then paid out.
Many a gambler looks specifically for Slot machines that are well suited for "gambling". In this case, they usually look for slots that manage to keep the game balance at a high level for as long as possible through regular smaller wins. Those who want to fulfill the "wagering conditions" are usually not interested in slots where long losing streaks are more frequent. In this case, the previously granted bonus credit would quickly be completely gambled away.