You enter the names of your learners into the boxes, hit the Choose button, and Tombola will pick one at random. You can then ask your question, or otherwise engage with that learner. As they have no idea when they will be called upon (but can see they will definitely be picked), you avoid two common problems: Overly focusing on the more participatory students, and bypassing the quiet ones; and having less engaged students not ready themselves as they are betting on not being asked.
When you hit Choose again, another random selection is made, but avoids those already asked. This behaviour can be changed in the options- just click the cog icon in the toolbox (pink symbols in the upper right of the page). You can also turn the sound off here.
If you need more boxes, for more learners, hit the +1 button. Hovering over a box reveals the delete icons, click to remove that box.
A quick way to fill in the boxes, from, say, a spreadsheet or other electronic format, is to click the QuickFill button. You can copy and paste a list of users into what pops up.
Because doing this inevitably uses up some time best spent doing other things, you can save the groups you create for later recall. That’s what the up and down arrow icons are for. The data is saved within your web browser, and never goes anywhere near the server. That means I have no idea about your users. The downside is this means that when you move to a different browser, the groups won’t be there- but if you have a network login, you’ll hopefully be fine even if you work in multiple classrooms (if you have bookmarks/favourites that work wherever you are, so should Tombola).