Former real estate agent here

It means they have multiple offers at or above the owners expectation, and rather than going back and forth a thousand times, they give you one chance to make your best offer, the one with the best offer at the end of the process will, 'win'.

The way this works is that each agent will submit their offers to the listing agent, and the listing agent will present the best offer to the owner.

The downside to this process is dealing with an agent outside of the listing agent's team. Anyone outside of the team who makes a sale gets a cut (varies depending on the context of the situation) and so the listing agent is incentivised to help his buyers win and may give them, 'insider information' to help them win the highest and best (unless all of those bidding are his buyers, then the agent is indifferent with who purchases the house).

If you're dealing with an inexperienced agent outside of the listing agent's team, you're less likely to win the highest and best.

Hope that extra bit of information helps